Auto merge of #14347 - jcdyer:cdyer/len-parsing, r=Wafflespeanut

Only allow border-image-outset to use non-negative numbers.

Restricts border-image-outline to only allow positive values.

Fixes #14295

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #14295.

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components/style/properties/longhand/border.mako.rs

@@ -242,7 +242,7 @@ ${helpers.single_keyword("-moz-float-edge", "content-box margin-box",
     pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         let mut values = vec![];
         for _ in 0..4 {
-            let value = input.try(|input| LengthOrNumber::parse(input));
+            let value = input.try(|input| LengthOrNumber::parse_non_negative(input));
             match value {
                 Ok(val) => values.push(val),
                 Err(_) => break,