diff --git a/tests/addition.rs b/tests/addition.rs
index 4dfce74..dd392c5 100644
--- a/tests/addition.rs
+++ b/tests/addition.rs
@@ -434,3 +434,8 @@ test!(
     "a {\n  color: (0/0) + (0/0);\n}\n",
     "a {\n  color: NaN;\n}\n"
 );
+test!(null_plus_null, "a {\n  color: null + null;\n}\n", "");
+error!(
+    color_plus_number,
+    "a {\n  color: red + 1;\n}\n", r#"Error: Undefined operation "red + 1"."#
+);
diff --git a/tests/custom-property.rs b/tests/custom-property.rs
index 27cef26..7406f52 100644
--- a/tests/custom-property.rs
+++ b/tests/custom-property.rs
@@ -95,3 +95,10 @@ error!(
     }",
     r#"Error: Declarations whose names begin with "--" may not be nested"#
 );
+error!(
+    empty_value,
+    "a {
+        --color:#{null};
+    }",
+    "Error: Custom property values may not be empty."
+);
diff --git a/tests/division.rs b/tests/division.rs
index 1131b60..a52d16e 100644
--- a/tests/division.rs
+++ b/tests/division.rs
@@ -279,3 +279,11 @@ test!(
     "a {\n  color: foo / calc(1vh + 1px);\n}\n",
     "a {\n  color: foo/calc(1vh + 1px);\n}\n"
 );
+error!(
+    color_div_color,
+    "a {\n  color: red / green;\n}\n", r#"Error: Undefined operation "red / green"."#
+);
+error!(
+    color_div_number,
+    "a {\n  color: red / 1;\n}\n", r#"Error: Undefined operation "red / 1"."#
+);
diff --git a/tests/equality.rs b/tests/equality.rs
index 4b4e6f4..2b98c01 100644
--- a/tests/equality.rs
+++ b/tests/equality.rs
@@ -260,3 +260,10 @@ test!(
     }",
     "a {\n  color: false;\n}\n"
 );
+test!(
+    number_equality_is_fuzzy,
+    "a {
+      color: .9999999999999999999999999999999==.99999999999999999999999999999998;
+    }",
+    "a {\n  color: true;\n}\n"
+);
diff --git a/tests/extend.rs b/tests/extend.rs
index 13bc1b0..6765f9d 100644
--- a/tests/extend.rs
+++ b/tests/extend.rs
@@ -1965,6 +1965,13 @@ error!(
     }",
     ""
 );
+error!(
+    extend_complex_selector,
+    "a {
+        @extend a>b;
+    }",
+    "Error: complex selectors may not be extended."
+);
 
 // todo: extend_loop (massive test)
 // todo: extend tests in folders
diff --git a/tests/for.rs b/tests/for.rs
index 7d2fea9..cef16c6 100644
--- a/tests/for.rs
+++ b/tests/for.rs
@@ -233,3 +233,12 @@ error!(
     invalid_keyword_after_first_number,
     "@for $i from 1 FOO 3 {}", "Error: Expected \"to\" or \"through\"."
 );
+test!(
+    to_equals_from,
+    "@for $i from 5 to 5 {
+        a {
+            color: red;
+        }
+    }",
+    ""
+);
diff --git a/tests/functions.rs b/tests/functions.rs
index 92366b8..09d93b1 100644
--- a/tests/functions.rs
+++ b/tests/functions.rs
@@ -409,5 +409,14 @@ test!(
     }",
     "a {\n  color: before;\n}\n"
 );
+error!(
+    function_no_return,
+    "@function foo() {}
+
+    a {
+        color: foo();
+    }",
+    "Error: Function finished without @return."
+);
 
 // todo: return inside if, return inside while, return inside for
diff --git a/tests/imports.rs b/tests/imports.rs
index 057b0d7..4b6b852 100644
--- a/tests/imports.rs
+++ b/tests/imports.rs
@@ -283,6 +283,22 @@ fn imports_import_only_scss() {
     );
 }
 
+#[test]
+fn imports_sass_file() {
+    let mut fs = TestFs::new();
+
+    fs.add_file("a.sass", "a\n\tcolor: red\n");
+
+    let input = r#"
+        @import "a";
+    "#;
+
+    assert_eq!(
+        "a {\n  color: red;\n}\n",
+        &grass::from_string(input.to_string(), &grass::Options::default().fs(&fs)).expect(input)
+    );
+}
+
 #[test]
 fn imports_absolute_scss() {
     let mut fs = TestFs::new();
diff --git a/tests/keyframes.rs b/tests/keyframes.rs
index 30fb964..150f59e 100644
--- a/tests/keyframes.rs
+++ b/tests/keyframes.rs
@@ -300,6 +300,15 @@ test!(
     }",
     "@keyframes spinner-border {\n  to {\n    color: red;\n  }\n}\na {\n  color: red;\n}\n"
 );
+test!(
+    percent_selector_leading_plus,
+    "@keyframes foo {
+        +5% {
+            color: red;
+        }
+    }",
+    "@keyframes foo {\n  +5% {\n    color: red;\n  }\n}\n"
+);
 error!(
     invalid_escape_in_place_of_e,
     "@keyframes foo {
@@ -309,6 +318,15 @@ error!(
     }",
     r#"Error: expected "%"."#
 );
+error!(
+    percent_selector_no_number_after_e,
+    "@keyframes foo {
+        5e% {
+            color: red;
+        }
+    }",
+    r#"Error: Expected digit."#
+);
 
 // todo: span for this
 // @keyframes foo {
diff --git a/tests/mixins.rs b/tests/mixins.rs
index 817f059..929c717 100644
--- a/tests/mixins.rs
+++ b/tests/mixins.rs
@@ -694,3 +694,10 @@ error!(
     ",
     "Error: Mixin doesn't accept a content block."
 );
+error!(
+    disallows_content_block_to_builtin_mixin,
+    r#"@use "sass:meta";
+
+    @include meta.load-css("") {}"#,
+    "Error: Mixin doesn't accept a content block."
+);
diff --git a/tests/modulo.rs b/tests/modulo.rs
index 2ceed82..55f92a3 100644
--- a/tests/modulo.rs
+++ b/tests/modulo.rs
@@ -180,3 +180,7 @@ error!(
     "a {\n  color: calc(1rem + 1px) % calc(1rem + 1px);\n}\n",
     r#"Error: Undefined operation "calc(1rem + 1px) % calc(1rem + 1px)"."#
 );
+error!(
+    number_mod_color,
+    "a {\n  color: 5 % red;\n}\n", r#"Error: Undefined operation "5 % red"."#
+);
diff --git a/tests/plain-css-fn.rs b/tests/plain-css-fn.rs
index d738741..437ac06 100644
--- a/tests/plain-css-fn.rs
+++ b/tests/plain-css-fn.rs
@@ -81,5 +81,12 @@ test!(
     "a {\n  color: or(foo);\n}\n",
     "a {\n  color: or(foo);\n}\n"
 );
-
-// todo: @function and($a) {} a { color: and(foo) }
+error!(
+    denies_keyword_arguments_to_interpolated_function,
+    "a {\n  color: f#{o}o($a: red);\n}\n",
+    "Error: Plain CSS functions don't support keyword arguments."
+);
+error!(
+    denies_function_named_after_keyword,
+    "@function and($a) {}", "Error: Invalid function name."
+);
diff --git a/tests/plain-css.rs b/tests/plain-css.rs
index b456ce2..d70c318 100644
--- a/tests/plain-css.rs
+++ b/tests/plain-css.rs
@@ -45,3 +45,121 @@ test!(
     "a {\n  color: not 2;\n  color: not true;\n  color: not false;\n}\n",
     grass::Options::default().input_syntax(InputSyntax::Css)
 );
+error!(
+    denies_silent_comment,
+    "// silent",
+    "Error: Silent comments aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    denies_function_rule,
+    "@function foo() {
+        @return 2;
+    }",
+    "Error: This at-rule isn't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    denies_content_rule,
+    "@content",
+    "Error: This at-rule isn't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+test!(
+    allows_media_rule,
+    "@media (foo) {
+        a {
+            color: red;
+        }
+    }",
+    "@media (foo) {\n  a {\n    color: red;\n  }\n}\n",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+test!(
+    allows_var_empty_second_arg,
+    "a {
+        color: var(1, );
+    }",
+    "a {\n  color: var(1, );\n}\n",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_empty_second_arg_in_non_var_function,
+    "a {
+        color: foo(1, );
+    }",
+    "Error: Expected expression.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_if_function,
+    "a {
+        color: if(true, a, b);
+    }",
+    "Error: This function isn't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_map_get_function,
+    "a {
+        color: map-get(true, a, b);
+    }",
+    "Error: This function isn't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_plus_operator,
+    "a {
+        color: 1 + 2;
+    }",
+    "Error: Operators aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_parens,
+    "a {
+        color: (a b);
+    }",
+    "Error: Parentheses aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_variable_expr,
+    "a {
+        color: $a;
+    }",
+    "Error: Sass variables aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_parent_selector_expr,
+    "a {
+        color: &;
+    }",
+    "Error: The parent selector isn't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_unary_plus,
+    "a {
+        color: +(1);
+    }",
+    "Error: Operators aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+error!(
+    disallows_unary_minus,
+    "a {
+        color: -(1);
+    }",
+    "Error: Operators aren't allowed in plain CSS.",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
+test!(
+    allows_rgb_function,
+    "a {
+        color: rgb(true, a, b);
+    }",
+    "a {\n  color: rgb(true, a, b);\n}\n",
+    grass::Options::default().input_syntax(InputSyntax::Css)
+);
diff --git a/tests/special-functions.rs b/tests/special-functions.rs
index 4b41807..c9b274d 100644
--- a/tests/special-functions.rs
+++ b/tests/special-functions.rs
@@ -251,6 +251,35 @@ test!(
     "a {\n  color: calc(1 + #{c});\n}\n",
     "a {\n  color: calc(1 + c);\n}\n"
 );
+test!(
+    progid_no_colon,
+    "a {\n  color: progid;\n}\n",
+    "a {\n  color: progid;\n}\n"
+);
+test!(
+    progid_no_colon_with_args,
+    "a {\n  color: progid();\n}\n",
+    "a {\n  color: progid();\n}\n"
+);
+test!(
+    can_add_s_and_ms,
+    "a {\n  color: calc(1s + 1ms);\n}\n",
+    "a {\n  color: 1.001s;\n}\n"
+);
+test!(
+    can_add_hz_and_khz,
+    "a {\n  color: calc(1Hz + 1kHz);\n}\n",
+    "a {\n  color: 1001Hz;\n}\n"
+);
+test!(
+    can_add_dpi_and_dppx,
+    "a {\n  color: calc(1dpi + 1dppx);\n}\n",
+    "a {\n  color: 97dpi;\n}\n"
+);
+error!(
+    nothing_after_last_arg,
+    "a { color: calc(1 + 1", r#"Error: expected "+", "-", "*", "/", or ")"."#
+);
 error!(
     progid_nothing_after,
     "a { color: progid:", "Error: expected \"(\"."
diff --git a/tests/strings.rs b/tests/strings.rs
index 48bef7d..1f6fb81 100644
--- a/tests/strings.rs
+++ b/tests/strings.rs
@@ -239,3 +239,8 @@ test!(
     "a {\n  color: \"#foo\";\n}\n",
     "a {\n  color: \"#foo\";\n}\n"
 );
+test!(
+    escaped_newline_inside_string,
+    "a {\n  color: \"f\\\n oo\";\n}\n",
+    "a {\n  color: \"f oo\";\n}\n"
+);
diff --git a/tests/subtraction.rs b/tests/subtraction.rs
index 854caca..d25160a 100644
--- a/tests/subtraction.rs
+++ b/tests/subtraction.rs
@@ -284,14 +284,19 @@ test!(
 );
 test!(
     sub_nan_left,
-    "a {\n  left:0/0-0;\n}\n",
+    "a {\n  left: (0/0) - 0;\n}\n",
     "a {\n  left: NaN;\n}\n"
 );
 test!(
     sub_nan_right,
-    "a {\n  left:0-0/0;\n}\n",
+    "a {\n  left: 0 - (0/0);\n}\n",
     "a {\n  left: NaN;\n}\n"
 );
+test!(
+    true_minus_null,
+    "a {\n  color: true - null;\n}\n",
+    "a {\n  color: true-;\n}\n"
+);
 error!(
     number_minus_color,
     "a {\n  color: 1 - #abc;\n}\n", "Error: Undefined operation \"1 - #abc\"."
@@ -332,3 +337,11 @@ error!(
     num_minus_calculation,
     "a {color: 1 - calc(1rem + 1px);}", r#"Error: Undefined operation "1 - calc(1rem + 1px)"."#
 );
+error!(
+    color_minus_number,
+    "a {color: red - 1;}", r#"Error: Undefined operation "red - 1"."#
+);
+error!(
+    map_minus_null,
+    "a {color: inspect((a: b) - null);}", r#"Error: (a: b) isn't a valid CSS value."#
+);
diff --git a/tests/supports.rs b/tests/supports.rs
index e46718d..44072f6 100644
--- a/tests/supports.rs
+++ b/tests/supports.rs
@@ -137,3 +137,25 @@ test!(
     ""
 );
 test!(supports_empty_body, "@supports (a: b) {}", "");
+test!(
+    does_not_simplify_calculation_in_args,
+    "@supports (calc(1 + 1)) {
+        a {
+            color: red;
+        }
+    }",
+    "@supports (calc(1 + 1)) {\n  a {\n    color: red;\n  }\n}\n"
+);
+error!(
+    supports_inside_declaration_body,
+    "@mixin foo() {
+        @supports (foo) {}
+    }
+
+    a {
+        color: {
+            @include foo();
+        }
+    }",
+    "Error: Supports rules may not be used within nested declarations."
+);
diff --git a/tests/units.rs b/tests/units.rs
index ac40228..dd0172c 100644
--- a/tests/units.rs
+++ b/tests/units.rs
@@ -212,6 +212,16 @@ test!(
     "a {\n  color: unit((1rem/1px) / 1vh);\n}\n",
     "a {\n  color: \"rem/px*vh\";\n}\n"
 );
+test!(
+    complex_unit_empty_numerator_single_denom,
+    "a {\n  color: unit(1 / 1px);\n}\n",
+    "a {\n  color: \"px^-1\";\n}\n"
+);
+test!(
+    complex_unit_empty_numerator_multiple_denom,
+    "a {\n  color: unit(1 / (1px*1rem));\n}\n",
+    "a {\n  color: \"(px*rem)^-1\";\n}\n"
+);
 error!(
     display_single_div_with_none_numerator,
     "a {\n  color: (1 / 1em);\n}\n", "Error: 1em^-1 isn't a valid CSS value."
@@ -243,6 +253,47 @@ error!(
     display_single_div_with_none_numerator_percent,
     "a {\n  color: (35 / 7%);\n}\n", "Error: 5%^-1 isn't a valid CSS value."
 );
+test!(
+    /// Verify the display implementation of all special-cased units
+    render_units,
+    "a {
+        color: 1px;
+        color: 1mm;
+        color: 1in;
+        color: 1cm;
+        color: 1q;
+        color: 1pt;
+        color: 1pc;
+        color: 1em;
+        color: 1rem;
+        color: 1lh;
+        color: 1%;
+        color: 1ex;
+        color: 1ch;
+        color: 1cap;
+        color: 1ic;
+        color: 1rlh;
+        color: 1vw;
+        color: 1vh;
+        color: 1vmin;
+        color: 1vmax;
+        color: 1vi;
+        color: 1vb;
+        color: 1deg;
+        color: 1grad;
+        color: 1rad;
+        color: 1turn;
+        color: 1s;
+        color: 1ms;
+        color: 1Hz;
+        color: 1kHz;
+        color: 1dpi;
+        color: 1dpcm;
+        color: 1dppx;
+        color: 1fr;
+        color: 1foo;
+    }", "a {\n  color: 1px;\n  color: 1mm;\n  color: 1in;\n  color: 1cm;\n  color: 1q;\n  color: 1pt;\n  color: 1pc;\n  color: 1em;\n  color: 1rem;\n  color: 1lh;\n  color: 1%;\n  color: 1ex;\n  color: 1ch;\n  color: 1cap;\n  color: 1ic;\n  color: 1rlh;\n  color: 1vw;\n  color: 1vh;\n  color: 1vmin;\n  color: 1vmax;\n  color: 1vi;\n  color: 1vb;\n  color: 1deg;\n  color: 1grad;\n  color: 1rad;\n  color: 1turn;\n  color: 1s;\n  color: 1ms;\n  color: 1Hz;\n  color: 1kHz;\n  color: 1dpi;\n  color: 1dpcm;\n  color: 1dppx;\n  color: 1fr;\n  color: 1foo;\n}\n"
+);
 
 macro_rules! test_unit_addition {
     ($u1:ident, $u2:ident, $out:literal) => {
diff --git a/tests/unknown-at-rule.rs b/tests/unknown-at-rule.rs
index 3ed6d6b..91b631f 100644
--- a/tests/unknown-at-rule.rs
+++ b/tests/unknown-at-rule.rs
@@ -105,5 +105,18 @@ test!(
     "a {\n  @box-shadow : $btn-focus-box-shadow, // $btn-active-box-shadow;;\n}\n"
 );
 test!(contains_multiline_comment, "@foo /**/;\n", "@foo;\n");
+error!(
+    unknown_at_rule_inside_declaration_body,
+    "@mixin foo {
+        @foo;
+    }
+
+    a {
+        color: {
+            @include foo;
+        }
+    }",
+    "Error: At-rules may not be used within nested declarations."
+);
 
 // todo: test scoping in rule
diff --git a/tests/use.rs b/tests/use.rs
index 1db3072..7e8c489 100644
--- a/tests/use.rs
+++ b/tests/use.rs
@@ -52,6 +52,10 @@ error!(
     use_empty_string,
     r#"@use "";"#, r#"Error: The default namespace "" is not a valid Sass identifier."#
 );
+error!(
+    configure_builtin_module,
+    r#"@use "sass:math" with ($e: 5);"#, r#"Error: Built-in modules can't be configured."#
+);
 test!(
     use_as,
     "@use \"sass:math\" as foo;