[cssom] Serialization of custom identifiers

[ewilligers]

https://drafts.csswg.org/css-grid/#propdef-grid-template-areas

Note: These rules can produce cell names that do not match the syntax, such as "1st 2nd 3rd", which requires escaping when referencing those areas by name in other properties, like grid-row: \31st; to reference the area named 1st.

The following parsing tests show variation between browsers in how custom identifiers are serialized:

test_valid_value("grid-column-end",  "\\31st", "\\31 st"); // passes in Blink, Firefox
test_valid_value("grid-column-end",  "\\31st", "1st"); // passes in Edge
test_valid_value("grid-column-end",  "\\31st", '"1st"'); // passes in WebKit

None of these serializations are accepted by any of the main browsers as a property value: none of the browsers round-trip. If we would like to round trip, we would need to serialize as "\\31st".

[upsuper]

It accepts <custom-ident> which clearly indicates that Edge and WebKit are wrong (the value they serialize is invalid). And CSSOM where serialization is generally defined doesn't seem to contain anything about that. I guess we can get serialization of <custom-ident> defined to do the Blink / Gecko way.