allow units beginning with a single -

src/parse/value/parse.rs

@@ -476,6 +476,22 @@ impl<'a> Parser<'a> {
                     span = span.merge(u.span);
                     Unit::from(u.node)
                 }
+                '-' => {
+                    if let Some(Token { kind, .. }) = self.toks.peek_next().cloned() {
+                        self.toks.reset_cursor();
+                        if matches!(kind, 'a'..='z' | 'A'..='Z' | '_' | '\\' | '\u{7f}'..=std::char::MAX)
+                        {
+                            let u = self.parse_identifier_no_interpolation(true)?;
+                            span = span.merge(u.span);
+                            Unit::from(u.node)
+                        } else {
+                            Unit::None
+                        }
+                    } else {
+                        self.toks.reset_cursor();
+                        Unit::None
+                    }
+                }
                 '%' => {
                     span = span.merge(self.toks.next().unwrap().pos());
                     Unit::Percent

tests/units.rs

@@ -127,6 +127,31 @@ test!(
     "a {\n  color: unit((1 / 1in) * 1in);\n}\n",
     "a {\n  color: \"\";\n}\n"
 );
+test!(
+    unit_begins_with_single_hyphen,
+    "a {\n  color: unit(1-em);\n}\n",
+    "a {\n  color: \"-em\";\n}\n"
+);
+test!(
+    unit_begins_with_two_hyphens,
+    "a {\n  color: 1--em;\n}\n",
+    "a {\n  color: 1 --em;\n}\n"
+);
+test!(
+    unit_begins_with_escape_sequence,
+    "a {\n  color: unit(1\\65);\n}\n",
+    "a {\n  color: \"e\";\n}\n"
+);
+test!(
+    unit_begins_with_escape_sequence_followed_by_space_and_hyphen,
+    "a {\n  color: unit(1\\65 -);\n}\n",
+    "a {\n  color: \"e-\";\n}\n"
+);
+test!(
+    unit_begins_with_single_hyphen_followed_by_escape_sequence,
+    "a {\n  color: unit(1-\\65);\n}\n",
+    "a {\n  color: \"-e\";\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."