In part of a larger review of CSS semantics and behaviors, this patch
takes the opportunity to remove leading whitespace in an updated class
attribute after the first class in the attribute has been removed.
Previously, if the first class name had been removed, the whitespace
that formerly followed it would remain in the class attribute. This
stood in contrast to removing other class names, which removed their
associated whitespace.
There should be no semantic or functional changes in this patch, only
a slightly-large diff for modified HTML documents that looks prettier
when removing the first class name in a class attribute.
Developed in https://github.com/WordPress/wordpress-develop/pull/6933
Discussed in https://core.trac.wordpress.org/ticket/61531
Props dmsnell, jonsurrell.
See #61531.
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