Remove F87

[awkawk]

Removing F87 Failure technique. Related to #1554

[JAWS-test]

F3 may also be removed soon, because CSS graphics can also get an alternative text. Possibly we should still wait for sufficient screen reader support

[awkawk]

This came up again today in an audit. This Failure has a couple of fatal flaws:

1. WCAG does not require that users can disable CSS. See https://www.w3.org/TR/UNDERSTANDING-WCAG/conformance.html#uc-conformance-claims-head where it indicates that CSS is one of the technologies that a web might identify as "relied upon".
2. In the technique, it is indicating that sites cannot use before/after CSS content unless it is decorative. In the procedure 2 is "Verify that the content is decorative" (if this is false then the failure condition applies). This is counter to the conformance requirements. Procedure step 3 is redundant in that it only applies if step 2 is false, and the results indicate failure if 2 or 3 are false.

For a failure, it needs to represent a failure all of the time, and this clearly doesn't. We might say "browser X" doesn't support this, but WCAG doesn't require that every browser/AT provides support for any technology or technique: https://www.w3.org/TR/UNDERSTANDING-WCAG/conformance.html#uc-support-level-head

[patrickhlauke]

due to the number of conflicts on this, I will redo this as a fresh PR, but we agreed in today's TF discussion that this is valid