diff --git a/CHANGELOG.md b/CHANGELOG.md
index 765f947..cd1477d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,14 @@
 
 -->
 
+# 0.12.1 (unreleased)
+
+- improve error message for complex units in calculations
+- more accurate formatting of named arguments in arglists when passed to `inspect(..)`
+- support `$whiteness` and `$blackness` as arguments to `scale-color(..)`
+- more accurate list separator from `join(..)`
+- resolve unicode edge cases in `str-index(..)`
+
 # 0.12.0
 
 - complete rewrite of parsing, evaluation, and serialization steps
diff --git a/README.md b/README.md
index 06190ec..1d63ccc 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ a bug except for in the case of error messages and error spans.
 
 Every commit of `grass` is tested against bootstrap v5.0.2, and every release is tested against the last 2,500 commits of bootstrap's `main` branch.
 
-That said, there are a number of known missing features and bugs. The rough edges of `grass` largely include `@forward` and more complex uses of `@uses`. We support basic usage of these rules, but more advanced features such as `@import`ing modules containing `@forward` with prefixes may not behave as expected.
+That said, there are a number of known missing features and bugs. The rough edges of `grass` largely include `@forward` and more complex uses of `@use`. We support basic usage of these rules, but more advanced features such as `@import`ing modules containing `@forward` with prefixes may not behave as expected.
 
 All known missing features and bugs are tracked in [#19](https://github.com/connorskees/grass/issues/19).
 
@@ -71,8 +71,8 @@ Using a modified version of the spec runner that ignores warnings and error span
 
 ```
 2022-12-26
-PASSING: 6024
-FAILING: 881
+PASSING: 6077
+FAILING: 828
 TOTAL: 6905
 ```
 
diff --git a/src/builtin/functions/color/hwb.rs b/src/builtin/functions/color/hwb.rs
index a1f8326..cceb0ea 100644
--- a/src/builtin/functions/color/hwb.rs
+++ b/src/builtin/functions/color/hwb.rs
@@ -9,11 +9,8 @@ pub(crate) fn blackness(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
         .get_err(0, "color")?
         .assert_color_with_name("color", args.span())?;
 
-    let blackness =
-        Number(1.0) - (color.red().max(color.green()).max(color.blue()) / Number(255.0));
-
     Ok(Value::Dimension(SassNumber {
-        num: (blackness * 100),
+        num: color.blackness() * 100,
         unit: Unit::Percent,
         as_slash: None,
     }))
@@ -26,10 +23,8 @@ pub(crate) fn whiteness(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
         .get_err(0, "color")?
         .assert_color_with_name("color", args.span())?;
 
-    let whiteness = color.red().min(color.green()).min(color.blue()) / Number(255.0);
-
     Ok(Value::Dimension(SassNumber {
-        num: (whiteness * 100),
+        num: color.whiteness() * 100,
         unit: Unit::Percent,
         as_slash: None,
     }))
@@ -102,9 +97,11 @@ pub(crate) fn hwb(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult
             visitor,
             args.span(),
         )? {
-            ParsedChannels::String(s) => {
-                Err((format!("Expected numeric channels, got {}", s), args.span()).into())
-            }
+            ParsedChannels::String(s) => Err((
+                format!("Expected numeric channels, got \"{}\"", s),
+                args.span(),
+            )
+                .into()),
             ParsedChannels::List(list) => {
                 let args = ArgumentResult {
                     positional: list,
diff --git a/src/builtin/functions/color/other.rs b/src/builtin/functions/color/other.rs
index 57f8c3e..76a73df 100644
--- a/src/builtin/functions/color/other.rs
+++ b/src/builtin/functions/color/other.rs
@@ -79,15 +79,15 @@ pub(crate) fn change_color(mut args: ArgumentResult, visitor: &mut Visitor) -> S
     };
 
     opt_hsl!(args, saturation, "saturation", 0, 100);
-    opt_hsl!(args, luminance, "lightness", 0, 100);
+    opt_hsl!(args, lightness, "lightness", 0, 100);
 
-    if hue.is_some() || saturation.is_some() || luminance.is_some() {
+    if hue.is_some() || saturation.is_some() || lightness.is_some() {
         // Color::as_hsla() returns more exact values than Color::hue(), etc.
-        let (this_hue, this_saturation, this_luminance, this_alpha) = color.as_hsla();
+        let (this_hue, this_saturation, this_lightness, this_alpha) = color.as_hsla();
         return Ok(Value::Color(Box::new(Color::from_hsla(
             hue.unwrap_or(this_hue),
             saturation.unwrap_or(this_saturation),
-            luminance.unwrap_or(this_luminance),
+            lightness.unwrap_or(this_lightness),
             alpha.unwrap_or(this_alpha),
         ))));
     }
@@ -132,15 +132,15 @@ pub(crate) fn adjust_color(mut args: ArgumentResult, visitor: &mut Visitor) -> S
     };
 
     opt_hsl!(args, saturation, "saturation", -100, 100);
-    opt_hsl!(args, luminance, "lightness", -100, 100);
+    opt_hsl!(args, lightness, "lightness", -100, 100);
 
-    if hue.is_some() || saturation.is_some() || luminance.is_some() {
+    if hue.is_some() || saturation.is_some() || lightness.is_some() {
         // Color::as_hsla() returns more exact values than Color::hue(), etc.
-        let (this_hue, this_saturation, this_luminance, this_alpha) = color.as_hsla();
+        let (this_hue, this_saturation, this_lightness, this_alpha) = color.as_hsla();
         return Ok(Value::Color(Box::new(Color::from_hsla(
             this_hue + hue.unwrap_or_else(Number::zero),
             this_saturation + saturation.unwrap_or_else(Number::zero),
-            this_luminance + luminance.unwrap_or_else(Number::zero),
+            this_lightness + lightness.unwrap_or_else(Number::zero),
             this_alpha + alpha.unwrap_or_else(Number::zero),
         ))));
     }
@@ -163,47 +163,39 @@ pub(crate) fn scale_color(mut args: ArgumentResult, visitor: &mut Visitor) -> Sa
         val + (if by.is_positive() { max - val } else { val }) * by
     }
 
+    fn check_num(num: Spanned<Value>, name: &str, min: f64, max: f64) -> SassResult<Number> {
+        let span = num.span;
+        let mut num = num.node.assert_number_with_name(name, span)?;
+
+        num.assert_unit(&Unit::Percent, name, span)?;
+        num.assert_bounds(name, min, max, span)?;
+
+        num.num /= Number(100.0);
+
+        Ok(num.num)
+    }
+
+    fn get_arg(
+        args: &mut ArgumentResult,
+        name: &str,
+        min: f64,
+        max: f64,
+    ) -> SassResult<Option<Number>> {
+        Ok(match args.get(usize::MAX, name) {
+            Some(v) => Some(check_num(v, name, min, max)?),
+            None => None,
+        })
+    }
+
     let span = args.span();
     let color = args
         .get_err(0, "color")?
         .assert_color_with_name("color", args.span())?;
 
-    macro_rules! opt_scale_arg {
-        ($args:ident, $name:ident, $arg:literal, $low:literal, $high:literal) => {
-            let $name = match $args.default_named_arg($arg, Value::Null) {
-                Value::Dimension(SassNumber { num: n, .. }) if n.is_nan() => todo!(),
-                Value::Dimension(SassNumber {
-                    num: n,
-                    unit: Unit::Percent,
-                    ..
-                }) => Some(bound!($args, $arg, n, Unit::Percent, $low, $high) / Number(100.0)),
-                v @ Value::Dimension { .. } => {
-                    return Err((
-                        format!(
-                            "${}: Expected {} to have unit \"%\".",
-                            $arg,
-                            v.inspect($args.span())?
-                        ),
-                        $args.span(),
-                    )
-                        .into())
-                }
-                Value::Null => None,
-                v => {
-                    return Err((
-                        format!("${}: {} is not a number.", $arg, v.inspect($args.span())?),
-                        $args.span(),
-                    )
-                        .into())
-                }
-            };
-        };
-    }
-
-    opt_scale_arg!(args, alpha, "alpha", -100, 100);
-    opt_scale_arg!(args, red, "red", -100, 100);
-    opt_scale_arg!(args, green, "green", -100, 100);
-    opt_scale_arg!(args, blue, "blue", -100, 100);
+    let red = get_arg(&mut args, "red", -100.0, 100.0)?;
+    let green = get_arg(&mut args, "green", -100.0, 100.0)?;
+    let blue = get_arg(&mut args, "blue", -100.0, 100.0)?;
+    let alpha = get_arg(&mut args, "alpha", -100.0, 100.0)?;
 
     if red.is_some() || green.is_some() || blue.is_some() {
         return Ok(Value::Color(Box::new(Color::from_rgba(
@@ -226,12 +218,12 @@ pub(crate) fn scale_color(mut args: ArgumentResult, visitor: &mut Visitor) -> Sa
         ))));
     }
 
-    opt_scale_arg!(args, saturation, "saturation", -100, 100);
-    opt_scale_arg!(args, luminance, "lightness", -100, 100);
+    let saturation = get_arg(&mut args, "saturation", -100.0, 100.0)?;
+    let lightness = get_arg(&mut args, "lightness", -100.0, 100.0)?;
 
-    if saturation.is_some() || luminance.is_some() {
+    if saturation.is_some() || lightness.is_some() {
         // Color::as_hsla() returns more exact values than Color::hue(), etc.
-        let (this_hue, this_saturation, this_luminance, this_alpha) = color.as_hsla();
+        let (this_hue, this_saturation, this_lightness, this_alpha) = color.as_hsla();
         return Ok(Value::Color(Box::new(Color::from_hsla(
             scale(this_hue, Number::zero(), Number(360.0)),
             scale(
@@ -240,8 +232,8 @@ pub(crate) fn scale_color(mut args: ArgumentResult, visitor: &mut Visitor) -> Sa
                 Number::one(),
             ),
             scale(
-                this_luminance,
-                luminance.unwrap_or_else(Number::zero),
+                this_lightness,
+                lightness.unwrap_or_else(Number::zero),
                 Number::one(),
             ),
             scale(
@@ -252,6 +244,34 @@ pub(crate) fn scale_color(mut args: ArgumentResult, visitor: &mut Visitor) -> Sa
         ))));
     }
 
+    let whiteness = get_arg(&mut args, "whiteness", -100.0, 100.0)?;
+    let blackness = get_arg(&mut args, "blackness", -100.0, 100.0)?;
+
+    if whiteness.is_some() || blackness.is_some() {
+        let this_hue = color.hue();
+        let this_whiteness = color.whiteness() * Number(100.0);
+        let this_blackness = color.blackness() * Number(100.0);
+
+        return Ok(Value::Color(Box::new(Color::from_hwb(
+            scale(this_hue, Number::zero(), Number(360.0)),
+            scale(
+                this_whiteness,
+                whiteness.unwrap_or_else(Number::zero),
+                Number(100.0),
+            ),
+            scale(
+                this_blackness,
+                blackness.unwrap_or_else(Number::zero),
+                Number(100.0),
+            ),
+            scale(
+                color.alpha(),
+                alpha.unwrap_or_else(Number::zero),
+                Number::one(),
+            ),
+        ))));
+    }
+
     Ok(Value::Color(if let Some(a) = alpha {
         let temp_alpha = color.alpha();
         Box::new(color.with_alpha(scale(temp_alpha, a, Number::one())))
diff --git a/src/builtin/functions/list.rs b/src/builtin/functions/list.rs
index 32dfc7b..012f06e 100644
--- a/src/builtin/functions/list.rs
+++ b/src/builtin/functions/list.rs
@@ -77,7 +77,7 @@ pub(crate) fn set_nth(mut args: ArgumentResult, visitor: &mut Visitor) -> SassRe
             Brackets::None,
         ),
         Value::Map(m) => (m.as_list(), ListSeparator::Comma, Brackets::None),
-        v => (vec![v], ListSeparator::Space, Brackets::None),
+        v => (vec![v], ListSeparator::Undecided, Brackets::None),
     };
     let (n, unit) = match args.get_err(1, "n")? {
         Value::Dimension(SassNumber {
@@ -133,7 +133,7 @@ pub(crate) fn append(mut args: ArgumentResult, visitor: &mut Visitor) -> SassRes
     args.max_args(3)?;
     let (mut list, sep, brackets) = match args.get_err(0, "list")? {
         Value::List(v, sep, b) => (v, sep, b),
-        v => (vec![v], ListSeparator::Space, Brackets::None),
+        v => (vec![v], ListSeparator::Undecided, Brackets::None),
     };
     let val = args.get_err(1, "val")?;
     let sep = match args.default_arg(
@@ -142,7 +142,13 @@ pub(crate) fn append(mut args: ArgumentResult, visitor: &mut Visitor) -> SassRes
         Value::String("auto".to_owned(), QuoteKind::None),
     ) {
         Value::String(s, ..) => match s.as_str() {
-            "auto" => sep,
+            "auto" => {
+                if sep == ListSeparator::Undecided {
+                    ListSeparator::Space
+                } else {
+                    sep
+                }
+            }
             "comma" => ListSeparator::Comma,
             "space" => ListSeparator::Space,
             "slash" => ListSeparator::Slash,
@@ -173,12 +179,12 @@ pub(crate) fn join(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResul
     let (mut list1, sep1, brackets) = match args.get_err(0, "list1")? {
         Value::List(v, sep, brackets) => (v, sep, brackets),
         Value::Map(m) => (m.as_list(), ListSeparator::Comma, Brackets::None),
-        v => (vec![v], ListSeparator::Space, Brackets::None),
+        v => (vec![v], ListSeparator::Undecided, Brackets::None),
     };
     let (list2, sep2) = match args.get_err(1, "list2")? {
         Value::List(v, sep, ..) => (v, sep),
         Value::Map(m) => (m.as_list(), ListSeparator::Comma),
-        v => (vec![v], ListSeparator::Space),
+        v => (vec![v], ListSeparator::Undecided),
     };
     let sep = match args.default_arg(
         2,
@@ -187,10 +193,12 @@ pub(crate) fn join(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResul
     ) {
         Value::String(s, ..) => match s.as_str() {
             "auto" => {
-                if list1.is_empty() || (list1.len() == 1 && sep1 == ListSeparator::Space) {
+                if sep1 != ListSeparator::Undecided {
+                    sep1
+                } else if sep2 != ListSeparator::Undecided {
                     sep2
                 } else {
-                    sep1
+                    ListSeparator::Space
                 }
             }
             "comma" => ListSeparator::Comma,
diff --git a/src/builtin/functions/string.rs b/src/builtin/functions/string.rs
index 3d8375c..52798e8 100644
--- a/src/builtin/functions/string.rs
+++ b/src/builtin/functions/string.rs
@@ -75,16 +75,9 @@ pub(crate) fn str_slice(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
 
     let span = args.span();
 
-    let (string, quotes) = match args.get_err(0, "string")? {
-        Value::String(s, q) => (s, q),
-        v => {
-            return Err((
-                format!("$string: {} is not a string.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let (string, quotes) = args
+        .get_err(0, "string")?
+        .assert_string_with_name("string", args.span())?;
 
     let str_len = string.chars().count();
 
@@ -141,63 +134,40 @@ pub(crate) fn str_slice(mut args: ArgumentResult, visitor: &mut Visitor) -> Sass
 
 pub(crate) fn str_index(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult<Value> {
     args.max_args(2)?;
-    let s1 = match args.get_err(0, "string")? {
-        Value::String(i, _) => i,
-        v => {
-            return Err((
-                format!("$string: {} is not a string.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
+    let s1 = args
+        .get_err(0, "string")?
+        .assert_string_with_name("string", args.span())?
+        .0;
+
+    let substr = args
+        .get_err(1, "substring")?
+        .assert_string_with_name("substring", args.span())?
+        .0;
+
+    let char_position = match s1.find(&substr) {
+        Some(i) => s1[0..i].chars().count() + 1,
+        None => return Ok(Value::Null),
     };
 
-    let substr = match args.get_err(1, "substring")? {
-        Value::String(i, _) => i,
-        v => {
-            return Err((
-                format!("$substring: {} is not a string.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
-
-    Ok(match s1.find(&substr) {
-        Some(v) => Value::Dimension(SassNumber {
-            num: (Number::from(v + 1)),
-            unit: Unit::None,
-            as_slash: None,
-        }),
-        None => Value::Null,
-    })
+    Ok(Value::Dimension(SassNumber {
+        num: Number::from(char_position),
+        unit: Unit::None,
+        as_slash: None,
+    }))
 }
 
 pub(crate) fn str_insert(mut args: ArgumentResult, visitor: &mut Visitor) -> SassResult<Value> {
     args.max_args(3)?;
     let span = args.span();
 
-    let (s1, quotes) = match args.get_err(0, "string")? {
-        Value::String(i, q) => (i, q),
-        v => {
-            return Err((
-                format!("$string: {} is not a string.", v.inspect(span)?),
-                span,
-            )
-                .into())
-        }
-    };
+    let (s1, quotes) = args
+        .get_err(0, "string")?
+        .assert_string_with_name("string", args.span())?;
 
-    let substr = match args.get_err(1, "insert")? {
-        Value::String(i, _) => i,
-        v => {
-            return Err((
-                format!("$insert: {} is not a string.", v.inspect(span)?),
-                span,
-            )
-                .into())
-        }
-    };
+    let substr = args
+        .get_err(1, "insert")?
+        .assert_string_with_name("insert", args.span())?
+        .0;
 
     let index = args
         .get_err(2, "index")?
diff --git a/src/builtin/modules/math.rs b/src/builtin/modules/math.rs
index a801ed5..a3a32a7 100644
--- a/src/builtin/modules/math.rs
+++ b/src/builtin/modules/math.rs
@@ -189,55 +189,20 @@ fn hypot(args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
 fn log(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(2)?;
 
-    let number = match args.get_err(0, "number")? {
-        // Value::Dimension { num: n, .. } if n.is_nan() => todo!(),
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => num,
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to be unitless.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let span = args.span();
+
+    let number = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", span)?;
+    number.assert_no_units("number", span)?;
+    let number = number.num;
 
     let base = match args.default_arg(1, "base", Value::Null) {
         Value::Null => None,
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => Some(num),
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to be unitless.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
         v => {
-            return Err((
-                format!("$base: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
+            let base = v.assert_number_with_name("base", span)?;
+            base.assert_no_units("base", span)?;
+            Some(base.num)
         }
     };
 
@@ -264,58 +229,20 @@ fn log(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
 fn pow(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(2)?;
 
-    let base = match args.get_err(0, "base")? {
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => num,
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$base: Expected {} to have no units.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$base: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let span = args.span();
 
-    let exponent = match args.get_err(1, "exponent")? {
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => num,
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$exponent: Expected {} to have no units.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$exponent: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    };
+    let base = args
+        .get_err(0, "base")?
+        .assert_number_with_name("base", span)?;
+    base.assert_no_units("base", span)?;
+
+    let exponent = args
+        .get_err(1, "exponent")?
+        .assert_number_with_name("exponent", span)?;
+    exponent.assert_no_units("exponent", span)?;
 
     Ok(Value::Dimension(SassNumber {
-        num: base.pow(exponent),
+        num: base.num.pow(exponent.num),
         unit: Unit::None,
         as_slash: None,
     }))
@@ -323,36 +250,16 @@ fn pow(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
 
 fn sqrt(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    let number = args.get_err(0, "number")?;
+    let number = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", args.span())?;
+    number.assert_no_units("number", args.span())?;
 
-    Ok(match number {
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => Value::Dimension(SassNumber {
-            num: num.sqrt(),
-            unit: Unit::None,
-            as_slash: None,
-        }),
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to have no units.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    })
+    Ok(Value::Dimension(SassNumber {
+        num: number.num.sqrt(),
+        unit: Unit::None,
+        as_slash: None,
+    }))
 }
 
 macro_rules! trig_fn {
@@ -400,136 +307,83 @@ trig_fn!(tan);
 
 fn acos(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    let number = args.get_err(0, "number")?;
 
-    Ok(match number {
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => Value::Dimension(SassNumber {
-            num: if num > Number(1.0) || num < Number(-1.0) {
-                Number(f64::NAN)
-            } else if num.is_one() {
-                Number::zero()
-            } else {
-                num.acos()
-            },
-            unit: Unit::Deg,
-            as_slash: None,
-        }),
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to be unitless.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    })
+    let span = args.span();
+
+    let number = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", span)?;
+    number.assert_no_units("number", span)?;
+    let number = number.num;
+
+    Ok(Value::Dimension(SassNumber {
+        num: if number > Number(1.0) || number < Number(-1.0) {
+            Number(f64::NAN)
+        } else if number.is_one() {
+            Number::zero()
+        } else {
+            number.acos()
+        },
+        unit: Unit::Deg,
+        as_slash: None,
+    }))
 }
 
 fn asin(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    let number = args.get_err(0, "number")?;
 
-    Ok(match number {
-        Value::Dimension(SassNumber {
-            num,
-            unit: Unit::None,
-            ..
-        }) => {
-            if num > Number(1.0) || num < Number(-1.0) {
-                return Ok(Value::Dimension(SassNumber {
-                    num: Number(f64::NAN),
-                    unit: Unit::Deg,
-                    as_slash: None,
-                }));
-            } else if num.is_zero() {
-                return Ok(Value::Dimension(SassNumber {
-                    num: Number::zero(),
-                    unit: Unit::Deg,
-                    as_slash: None,
-                }));
-            }
+    let span = args.span();
 
-            Value::Dimension(SassNumber {
-                num: num.asin(),
-                unit: Unit::Deg,
-                as_slash: None,
-            })
-        }
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to be unitless.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    })
+    let number = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", span)?;
+    number.assert_no_units("number", span)?;
+    let number = number.num;
+
+    if number > Number(1.0) || number < Number(-1.0) {
+        return Ok(Value::Dimension(SassNumber {
+            num: Number(f64::NAN),
+            unit: Unit::Deg,
+            as_slash: None,
+        }));
+    } else if number.is_zero() {
+        return Ok(Value::Dimension(SassNumber {
+            num: Number::zero(),
+            unit: Unit::Deg,
+            as_slash: None,
+        }));
+    }
+
+    Ok(Value::Dimension(SassNumber {
+        num: number.asin(),
+        unit: Unit::Deg,
+        as_slash: None,
+    }))
 }
 
 fn atan(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
     args.max_args(1)?;
-    let number = args.get_err(0, "number")?;
 
-    Ok(match number {
-        Value::Dimension(SassNumber {
-            num: n,
-            unit: Unit::None,
-            ..
-        }) => {
-            if n.is_zero() {
-                return Ok(Value::Dimension(SassNumber {
-                    num: (Number::zero()),
-                    unit: Unit::Deg,
-                    as_slash: None,
-                }));
-            }
+    let span = args.span();
 
-            Value::Dimension(SassNumber {
-                num: n.atan(),
-                unit: Unit::Deg,
-                as_slash: None,
-            })
-        }
-        v @ Value::Dimension(SassNumber { .. }) => {
-            return Err((
-                format!(
-                    "$number: Expected {} to be unitless.",
-                    v.inspect(args.span())?
-                ),
-                args.span(),
-            )
-                .into())
-        }
-        v => {
-            return Err((
-                format!("$number: {} is not a number.", v.inspect(args.span())?),
-                args.span(),
-            )
-                .into())
-        }
-    })
+    let number = args
+        .get_err(0, "number")?
+        .assert_number_with_name("number", span)?;
+    number.assert_no_units("number", span)?;
+
+    if number.num.is_zero() {
+        return Ok(Value::Dimension(SassNumber {
+            num: (Number::zero()),
+            unit: Unit::Deg,
+            as_slash: None,
+        }));
+    }
+
+    Ok(Value::Dimension(SassNumber {
+        num: number.num.atan(),
+        unit: Unit::Deg,
+        as_slash: None,
+    }))
 }
 
 fn atan2(mut args: ArgumentResult, _: &mut Visitor) -> SassResult<Value> {
diff --git a/src/color/mod.rs b/src/color/mod.rs
index fab61ce..2afa55c 100644
--- a/src/color/mod.rs
+++ b/src/color/mod.rs
@@ -505,4 +505,12 @@ impl Color {
 
         Color::new_rgba(red, green, blue, alpha, ColorFormat::Infer)
     }
+
+    pub fn whiteness(&self) -> Number {
+        self.red().min(self.green()).min(self.blue()) / Number(255.0)
+    }
+
+    pub fn blackness(&self) -> Number {
+        Number(1.0) - (self.red().max(self.green()).max(self.blue()) / Number(255.0))
+    }
 }
diff --git a/src/common.rs b/src/common.rs
index 1e78a5a..9394c2b 100644
--- a/src/common.rs
+++ b/src/common.rs
@@ -98,7 +98,8 @@ impl PartialEq for ListSeparator {
     #[allow(clippy::match_like_matches_macro)]
     fn eq(&self, other: &Self) -> bool {
         match (self, other) {
-            (Self::Space | Self::Undecided, Self::Space | Self::Undecided) => true,
+            (Self::Space, Self::Space) => true,
+            (Self::Undecided, Self::Undecided) => true,
             (Self::Comma, Self::Comma) => true,
             _ => false,
         }
diff --git a/src/evaluate/bin_op.rs b/src/evaluate/bin_op.rs
index ae2e042..31f6bd5 100644
--- a/src/evaluate/bin_op.rs
+++ b/src/evaluate/bin_op.rs
@@ -1,5 +1,3 @@
-#![allow(unused_variables)]
-
 use std::cmp::Ordering;
 
 use codemap::Span;
@@ -350,7 +348,7 @@ pub(crate) fn sub(left: Value, right: Value, options: &Options, span: Span) -> S
     })
 }
 
-pub(crate) fn mul(left: Value, right: Value, options: &Options, span: Span) -> SassResult<Value> {
+pub(crate) fn mul(left: Value, right: Value, _: &Options, span: Span) -> SassResult<Value> {
     Ok(match left {
         Value::Dimension(SassNumber {
             num,
@@ -412,7 +410,7 @@ pub(crate) fn mul(left: Value, right: Value, options: &Options, span: Span) -> S
 pub(crate) fn cmp(
     left: &Value,
     right: &Value,
-    options: &Options,
+    _: &Options,
     span: Span,
     op: BinaryOp,
 ) -> SassResult<Value> {
@@ -469,7 +467,7 @@ pub(crate) fn div(left: Value, right: Value, options: &Options, span: Span) -> S
             Value::Dimension(SassNumber {
                 num: num2,
                 unit: unit2,
-                as_slash: as_slash2,
+                ..
             }) => {
                 if unit2 == Unit::None {
                     return Ok(Value::Dimension(SassNumber {
@@ -540,7 +538,7 @@ pub(crate) fn div(left: Value, right: Value, options: &Options, span: Span) -> S
     })
 }
 
-pub(crate) fn rem(left: Value, right: Value, options: &Options, span: Span) -> SassResult<Value> {
+pub(crate) fn rem(left: Value, right: Value, _: &Options, span: Span) -> SassResult<Value> {
     Ok(match left {
         Value::Dimension(SassNumber {
             num: n,
diff --git a/src/serializer.rs b/src/serializer.rs
index c038f78..9866054 100644
--- a/src/serializer.rs
+++ b/src/serializer.rs
@@ -76,6 +76,15 @@ pub(crate) fn serialize_number(
     Ok(serializer.finish_for_expr())
 }
 
+pub(crate) fn inspect_float(number: f64, options: &Options, span: Span) -> String {
+    let map = CodeMap::new();
+    let mut serializer = Serializer::new(options, &map, true, span);
+
+    serializer.write_float(number);
+
+    serializer.finish_for_expr()
+}
+
 pub(crate) fn inspect_number(
     number: &SassNumber,
     options: &Options,
diff --git a/src/value/arglist.rs b/src/value/arglist.rs
index af0c75b..4d0a069 100644
--- a/src/value/arglist.rs
+++ b/src/value/arglist.rs
@@ -44,7 +44,7 @@ impl ArgList {
     }
 
     pub fn len(&self) -> usize {
-        self.elems.len() + self.keywords.len()
+        self.elems.len()
     }
 
     pub fn is_empty(&self) -> bool {
@@ -52,7 +52,6 @@ impl ArgList {
     }
 
     pub fn is_null(&self) -> bool {
-        // todo: include keywords
         !self.is_empty() && (self.elems.iter().all(Value::is_null))
     }
 
diff --git a/src/value/calculation.rs b/src/value/calculation.rs
index c938b3c..1706730 100644
--- a/src/value/calculation.rs
+++ b/src/value/calculation.rs
@@ -270,7 +270,7 @@ impl SassCalculation {
                             return Err((
                                 format!(
                                     "Number {} isn't compatible with CSS calculations.",
-                                    value.to_css_string(span, false)?
+                                    value.inspect(span)?
                                 ),
                                 span,
                             )
diff --git a/src/value/mod.rs b/src/value/mod.rs
index a98c6fd..742e878 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -236,6 +236,25 @@ impl Value {
         }
     }
 
+    pub fn assert_string_with_name(
+        self,
+        name: &str,
+        span: Span,
+    ) -> SassResult<(String, QuoteKind)> {
+        match self {
+            Value::String(s, quotes) => Ok((s, quotes)),
+            _ => Err((
+                format!(
+                    "${name}: {} is not a string.",
+                    self.inspect(span)?,
+                    name = name,
+                ),
+                span,
+            )
+                .into()),
+        }
+    }
+
     // todo: rename is_blank
     pub fn is_null(&self) -> bool {
         match self {
@@ -599,8 +618,8 @@ impl Value {
                     .join(", ")
             )),
             Value::Dimension(n) => Cow::Owned(inspect_number(n, &Options::default(), span)?),
-            Value::ArgList(args) if args.is_empty() => Cow::Borrowed("()"),
-            Value::ArgList(args) if args.len() == 1 => Cow::Owned(format!(
+            Value::ArgList(args) if args.elems.is_empty() => Cow::Borrowed("()"),
+            Value::ArgList(args) if args.elems.len() == 1 => Cow::Owned(format!(
                 "({},)",
                 args.elems
                     .iter()
diff --git a/src/value/sass_number.rs b/src/value/sass_number.rs
index a498fd4..8085270 100644
--- a/src/value/sass_number.rs
+++ b/src/value/sass_number.rs
@@ -4,7 +4,7 @@ use codemap::Span;
 
 use crate::{
     error::SassResult,
-    serializer::inspect_number,
+    serializer::{inspect_float, inspect_number},
     unit::{are_any_convertible, known_compatibilities_by_unit, Unit, UNIT_CONVERSION_TABLE},
     Options,
 };
@@ -162,6 +162,26 @@ impl SassNumber {
         }
     }
 
+    pub fn assert_bounds(&self, name: &str, min: f64, max: f64, span: Span) -> SassResult<()> {
+        if !(self.num <= Number(max) && self.num >= Number(min)) {
+            return Err((
+                format!(
+                    "${}: Expected {} to be within {}{} and {}{}.",
+                    name,
+                    inspect_number(self, &Options::default(), span)?,
+                    inspect_float(min, &Options::default(), span),
+                    self.unit,
+                    inspect_float(max, &Options::default(), span),
+                    self.unit,
+                ),
+                span,
+            )
+                .into());
+        }
+
+        Ok(())
+    }
+
     pub fn is_comparable_to(&self, other: &Self) -> bool {
         self.unit.comparable(&other.unit)
     }
diff --git a/tests/arglist.rs b/tests/arglist.rs
index cf0912c..115607d 100644
--- a/tests/arglist.rs
+++ b/tests/arglist.rs
@@ -92,3 +92,51 @@ test!(
     }",
     ""
 );
+test!(
+    keyword_args_no_positional,
+    "@mixin foo($a...) {
+        pos: inspect($a);
+        kw: inspect(keywords($a));
+    }
+
+    a {
+        @include foo($a: b);
+    }",
+    "a {\n  pos: ();\n  kw: (a: b);\n}\n"
+);
+test!(
+    keyword_args_one_positional,
+    "@mixin foo($a...) {
+        pos: inspect($a);
+        kw: inspect(keywords($a));
+    }
+
+    a {
+        @include foo(a, $b: c);
+    }",
+    "a {\n  pos: (a,);\n  kw: (b: c);\n}\n"
+);
+test!(
+    keyword_args_length_no_positional,
+    "@mixin foo($a...) {
+        pos: length($a);
+        kw: length(keywords($a));
+    }
+
+    a {
+        @include foo($a: b);
+    }",
+    "a {\n  pos: 0;\n  kw: 1;\n}\n"
+);
+error!(
+    keyword_args_no_positional_is_invalid,
+    "@mixin foo($a...) {
+        pos: $a;
+        kw: length(keywords($a));
+    }
+
+    a {
+        @include foo($a: b);
+    }",
+    "Error: () isn't a valid CSS value."
+);
diff --git a/tests/color_hsl.rs b/tests/color_hsl.rs
index f0b62b7..f27d973 100644
--- a/tests/color_hsl.rs
+++ b/tests/color_hsl.rs
@@ -307,3 +307,37 @@ test!(
     "a {\n  color: hue(adjust-hue(hsla(200, 50%, 50%), (0/0)));\n}\n",
     "a {\n  color: NaNdeg;\n}\n"
 );
+test!(
+    hsl_special_two_arg_var_first,
+    "a {\n  color: hsl(var(--foo), --bar);\n}\n",
+    "a {\n  color: hsl(var(--foo), --bar);\n}\n"
+);
+test!(
+    hsl_special_two_arg_var_second,
+    "a {\n  color: hsl(--foo, var(--bar));\n}\n",
+    "a {\n  color: hsl(--foo, var(--bar));\n}\n"
+);
+error!(
+    hsl_special_two_arg_neither_var,
+    "a {\n  color: hsl(--foo, --bar);\n}\n", "Error: Missing argument $lightness."
+);
+test!(
+    hsl_special_four_arg_var_last,
+    "a {\n  color: hsl(a, b, c, var(--bar));\n}\n",
+    "a {\n  color: hsl(a, b, c, var(--bar));\n}\n"
+);
+test!(
+    hsl_special_three_arg_var_last,
+    "a {\n  color: hsl(a, b, var(--bar));\n}\n",
+    "a {\n  color: hsl(a, b, var(--bar));\n}\n"
+);
+test!(
+    darken_all_channels_equal,
+    "a {\n  color: darken(#fff, 10);\n}\n",
+    "a {\n  color: #e6e6e6;\n}\n"
+);
+test!(
+    darken_green_channel_max,
+    "a {\n  color: darken(rgb(50, 200, 100), 10);\n}\n",
+    "a {\n  color: #289f50;\n}\n"
+);
diff --git a/tests/color_hwb.rs b/tests/color_hwb.rs
index a6c161e..876f65a 100644
--- a/tests/color_hwb.rs
+++ b/tests/color_hwb.rs
@@ -86,6 +86,11 @@ test!(
     "@use \"sass:color\";\na {\n  color: color.hwb(180 30% 40% / 0);\n}\n",
     "a {\n  color: rgba(77, 153, 153, 0);\n}\n"
 );
+test!(
+    scale_whiteness,
+    "a {\n  color: scale-color(#cc6666, $whiteness: 100%);\n}\n",
+    "a {\n  color: #d5d5d5;\n}\n"
+);
 error!(
     hwb_whiteness_missing_pct,
     "@use \"sass:color\";\na {\n  color: color.hwb(0, 0, 100);\n}\n",
diff --git a/tests/each.rs b/tests/each.rs
index ca0474a..a43b5cf 100644
--- a/tests/each.rs
+++ b/tests/each.rs
@@ -99,3 +99,7 @@ error!(
     missing_closing_curly_brace,
     "@each $i in 1 {", "Error: expected \"}\"."
 );
+error!(
+    in_has_characters_after,
+    "@each $i inaa 0 1 2 {}", "Error: Expected \"in\"."
+);
diff --git a/tests/extend.rs b/tests/extend.rs
index 759a549..c9c5331 100644
--- a/tests/extend.rs
+++ b/tests/extend.rs
@@ -1974,8 +1974,7 @@ error!(
 );
 error!(
     extend_at_root_of_document,
-    "@extend a;",
-    "Error: @extend may only be used within style rules."
+    "@extend a;", "Error: @extend may only be used within style rules."
 );
 
 // todo: extend_loop (massive test)
diff --git a/tests/forward.rs b/tests/forward.rs
index 622f062..d4f0109 100644
--- a/tests/forward.rs
+++ b/tests/forward.rs
@@ -215,4 +215,4 @@ error!(
         @forward "foo";
     "#,
     "Error: @forward rules must be written before any other rules."
-);
\ No newline at end of file
+);
diff --git a/tests/if.rs b/tests/if.rs
index 483bab2..0b9a5c7 100644
--- a/tests/if.rs
+++ b/tests/if.rs
@@ -228,11 +228,12 @@ error!(
     invalid_toplevel_selector,
     "@if true { & { } }", "Error: Top-level selectors may not contain the parent selector \"&\"."
 );
-error!(
-    #[ignore = "unsure what the exact rule is here wrt denying interpolation (@media allows this)"]
-    denies_interpolated_at_rule,
-    "@#{if} true { a {   color: red; } }", "Error: expected \"(\"."
+test!(
+    treats_interpolated_if_as_unknown_at_rule,
+    "@#{if} true { a {   color: red; } }",
+    "@if true {\n  a {\n    color: red;\n  }\n}\n"
 );
+
 test!(
     else_if_escaped_lower_i,
     r"@if false {
diff --git a/tests/imports.rs b/tests/imports.rs
index 4893912..a562750 100644
--- a/tests/imports.rs
+++ b/tests/imports.rs
@@ -485,6 +485,11 @@ test!(
     "@import \"foo.css\";",
     "@import \"foo.css\";\n"
 );
+test!(
+    newline_in_plain_css,
+    "@import \"fo\\\no.css\";",
+    "@import \"fo\\\no.css\";\n"
+);
 test!(import_url, "@import url(foo..);", "@import url(foo..);\n");
 test!(
     import_url_interpolation,
diff --git a/tests/list.rs b/tests/list.rs
index bd63c71..2d5a023 100644
--- a/tests/list.rs
+++ b/tests/list.rs
@@ -437,6 +437,18 @@ test!(
     "a {\n  color: a, A, \"Noto Color Emoji\";\n}\n",
     "a {\n  color: a, A, \"Noto Color Emoji\";\n}\n"
 );
+test!(
+    list_separator_of_empty_list_after_join,
+    "a {
+        color: list-separator(join(join((), (), comma), 1 2));
+        color: list-separator(join(join((), (), comma), (1, 2)));
+    }",
+    "a {\n  color: comma;\n  color: comma;\n}\n"
+);
+error!(
+    nth_list_index_0,
+    "a {\n  color: nth(a b c, 0);\n}\n", "Error: $n: List index may not be 0."
+);
 error!(
     invalid_item_in_space_separated_list,
     "a {\n  color: red color * #abc;\n}\n", "Error: Undefined operation \"color * #abc\"."
diff --git a/tests/math.rs b/tests/math.rs
index 04ecc99..fb0e525 100644
--- a/tests/math.rs
+++ b/tests/math.rs
@@ -126,3 +126,35 @@ error!(
     "a {\n  color: random(1000000000000000001 - 1000000000000000000);\n}\n",
     "Error: $limit: Must be greater than 0, was 0."
 );
+error!(
+    percentage_non_number_arg,
+    "a {\n  color: percentage(a);\n}\n", "Error: $number: a is not a number."
+);
+error!(
+    round_non_number_arg,
+    "a {\n  color: round(a);\n}\n", "Error: $number: a is not a number."
+);
+error!(
+    ceil_non_number_arg,
+    "a {\n  color: ceil(a);\n}\n", "Error: $number: a is not a number."
+);
+error!(
+    floor_non_number_arg,
+    "a {\n  color: floor(a);\n}\n", "Error: $number: a is not a number."
+);
+error!(
+    abs_non_number_arg,
+    "a {\n  color: abs(a);\n}\n", "Error: $number: a is not a number."
+);
+error!(
+    comparable_non_number_arg_both,
+    "a {\n  color: comparable(a, b);\n}\n", "Error: $number1: a is not a number."
+);
+error!(
+    comparable_non_number_arg_first,
+    "a {\n  color: comparable(a, 1);\n}\n", "Error: $number1: a is not a number."
+);
+error!(
+    comparable_non_number_arg_last,
+    "a {\n  color: comparable(1, b);\n}\n", "Error: $number2: b is not a number."
+);
diff --git a/tests/selectors.rs b/tests/selectors.rs
index accb7ff..dfeed8e 100644
--- a/tests/selectors.rs
+++ b/tests/selectors.rs
@@ -885,6 +885,48 @@ test!(
     }"#,
     "\\\\ {\n  color: \\\\;\n}\n"
 );
+test!(
+    pseudo_element_double_quotes,
+    r#"::foo("red") {
+        color: &;
+    }"#,
+    "::foo(\"red\") {\n  color: ::foo(\"red\");\n}\n"
+);
+test!(
+    pseudo_element_single_quotes,
+    r#"::foo('red') {
+        color: &;
+    }"#,
+    "::foo(\"red\") {\n  color: ::foo(\"red\");\n}\n"
+);
+test!(
+    pseudo_element_loud_comments,
+    r#"::foo(/**/a/**/b/**/) {
+        color: &;
+    }"#,
+    "::foo(a/**/b/**/) {\n  color: ::foo(a/**/b/**/);\n}\n"
+);
+test!(
+    pseudo_element_forward_slash,
+    r#"::foo(/a/b/) {
+        color: &;
+    }"#,
+    "::foo(/a/b/) {\n  color: ::foo(/a/b/);\n}\n"
+);
+error!(
+    pseudo_element_interpolated_semicolon_no_brackets,
+    r#"::foo(#{";"}) {
+        color: &;
+    }"#,
+    r#"Error: expected ")"."#
+);
+test!(
+    pseudo_element_interpolated_semicolon_with_parens,
+    r#"::foo((#{";"})) {
+        color: &;
+    }"#,
+    "::foo((;)) {\n  color: ::foo((;));\n}\n"
+);
 error!(
     a_n_plus_b_n_invalid_odd,
     ":nth-child(ofdd) {\n  color: &;\n}\n", "Error: Expected \"odd\"."
diff --git a/tests/special-functions.rs b/tests/special-functions.rs
index 221bab1..a7db4cf 100644
--- a/tests/special-functions.rs
+++ b/tests/special-functions.rs
@@ -42,6 +42,11 @@ error!(
     calc_retains_multiline_comment,
     "a {\n  color: calc(/**/);\n}\n", "Error: Expected number, variable, function, or calculation."
 );
+error!(
+    calc_complex_unit,
+    "a {\n  color: calc(1% + 1px * 2px);\n}\n",
+    "Error: Number 2px*px isn't compatible with CSS calculations."
+);
 error!(
     calc_nested_parens,
     "a {\n  color: calc((((()))));\n}\n",
diff --git a/tests/strings.rs b/tests/strings.rs
index 1f6fb81..bdc00ee 100644
--- a/tests/strings.rs
+++ b/tests/strings.rs
@@ -244,3 +244,13 @@ test!(
     "a {\n  color: \"f\\\n oo\";\n}\n",
     "a {\n  color: \"f oo\";\n}\n"
 );
+test!(
+    str_index_double_width_character,
+    "a {\n  color: str-index(\"👭a\", \"a\");\n}\n",
+    "a {\n  color: 2;\n}\n"
+);
+test!(
+    str_index_combining_character,
+    "a {\n  color: str-index(\"c\\0308 a\", \"a\");\n}\n",
+    "a {\n  color: 3;\n}\n"
+);