Auto merge of #18716 - jdm:parseSrcset, r=jdm

Parse srcset attribute values

Squashed version of #18313.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix (partially) #11416
- [x] There are tests for these changes

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components/style/str.rs

@@ -58,7 +58,8 @@ pub fn split_commas<'a>(s: &'a str) -> Filter<Split<'a, char>, fn(&&str) -> bool
     s.split(',').filter(not_empty as fn(&&str) -> bool)
 }
 
-fn is_ascii_digit(c: &char) -> bool {
+/// Character is ascii digit
+pub fn is_ascii_digit(c: &char) -> bool {
     match *c {
         '0'...'9' => true,
         _ => false,