#6233 Improve css validation

[martgil]

This PR improves currently implemented css validation on rendered elements in pgp_block.

Closes #6233

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Tests (delete all except exactly one):

• Does not need tests (refactor only, docs or internal changes)

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[martgil]

The failing test is not related to this PR but from a fragile test.

[sosnovsky]

perfect, thanks for the fixes!