Created 6 improved tests

[TalbotG]

active-selection-021.html
active-selection-021-ref.html
active-selection-024.html
active-selection-024-ref.html
active-selection-031.html
active-selection-031-ref.html
active-selection-041.html
active-selection-041-ref.html
active-selection-043.html
active-selection-045.html
support/60x60-red.png
support/60x60-green.png
support/100x100-red.png
reference/ref-filled-green-100px-square.xht
reference/ref-nothing-below.xht

This PR is a followup to
#20028

This PR contains all the corrections and improvements proposed in #20028

[TalbotG]

@fantasai , please review .

[TalbotG]

@fantasai ,

active-selection-024: improved test, first-letter

active-selection-025: new test, first-letter, no color declaration only background-color

active-selection-026: new test, first-line

active-selection-027: new test, first-line, no color declaration only background-color

These 4 tests take into account the resolution in
w3c/csswg-drafts#4625

[fantasai]

@TalbotG Remove the replaced element tests from this PR, because the rest of these are are acceptable tests, but 41 and 43 are not.

The transparency of the color wash is not dictated by the spec and each UA has their own color, so your approach is not valid. See #20028 (review) for an approach that will actually work to test this case. File a new, separate PR for the replaced element tests.

[fantasai]

See comment above.

[TalbotG]

I must close this PR and delete this branch now that
#24713
#24714
have merged.

I will create a distinct and separate PR for
active-selection-041.html
active-selection-041-ref.html
active-selection-043.html

I have bookmarked your
web-platform-tests wpt pull 20931#issuecomment-650434204