Need tests for non-empty white space only alt values, and interior white space values

[cookiecrook]

We have some image alt tests in this file:
https://github.com/web-platform-tests/wpt/blob/master/accname/name/comp_host_language_label.html

But I don’t recall testing the logic for generic/none role due to empty alt text (it may be in HTML-AAM) and I’m fairly certain we don’t have tests for non-empty white space variants like alt=" " and other values like TAB, CRLF, etc, which come out of CMS templates sometimes.

Also interior non-space white space should be normalized too.

<img src="…" alt="foo
bar" data-expectedlabel="foo bar">

[zcorpan]

A space should be different from the empty string, right?

[cookiecrook]

Correct, though there may be a sufficient requirement in the spec to trim a non-empty whitespace string down to the same equivalent empty string. In any case, I'd expect whitespace-only alt text to be an author error. Proposed tests are primarily to ensure all engines are doing the same thing with the author error.

[cookiecrook]

For img alt, HTML differentiates the empty string from any other value -- only the empty string is special.
https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element%3Athe-img-element-11

Originally posted by @zcorpan in #115 (comment)