Auto merge of #15278 - atheed:zero-parsing, r=Wafflespeanut

Parsing "0" as Number for line-height and border-image-outset

Fixes #15171 by correctly parsing `0` as `0` (rather than as `0px`, as was the case earlier) for the `line-height` and `border-image-outset` CSS properties. Wrote unit tests for both; `./mach test-unit -p style` passes all tests.

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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15171

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- [X] There are tests for these changes.

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components/style/values/specified/length.rs

@@ -1196,10 +1196,13 @@ pub type LengthOrNumber = Either<Length, Number>;
 impl LengthOrNumber {
     /// Parse a non-negative LengthOrNumber.
     pub fn parse_non_negative(input: &mut Parser) -> Result<Self, ()> {
-        if let Ok(v) = input.try(Length::parse_non_negative) {
-            Ok(Either::First(v))
+        // We try to parse as a Number first because, for cases like LengthOrNumber,
+        // we want "0" to be parsed as a plain Number rather than a Length (0px); this
+        // matches the behaviour of all major browsers
+        if let Ok(v) = input.try(Number::parse_non_negative) {
+            Ok(Either::Second(v))
         } else {
-            Number::parse_non_negative(input).map(Either::Second)
+            Length::parse_non_negative(input).map(Either::First)
         }
     }
 }