Do you want to know how the CSS WG works? Fantasai has written about:csswg, An Inside View of the CSS Working Group at W3C.
Authors are used to CSS allowing spaces almost everywhere, and will most likely expect them to work here as well. Also
calc() will either allow or require spaces, and
:nth-child()‘s syntax should be consistent with that.
Resolved:: Copy attribute selector wording from Selectors into CSS2.1 to resolve incompatibility.
$= all accept the empty string, but match nothing. Note that this overrides a previous resolution which stated that such attribute selectors are invalid.
Rationale:: These selectors are meaningless. This resolution matches existing implementations for
~=, which was introduced in CSS2.1, and makes
$=, which were introduced in Level 3, consistent with that.
see also Alan Gresley's comment which was posted in response to this resolution]
Resolved: A mismatched close brace in a style attribute is treated as an invalid token. It does not terminate the style attribute’s style data.