Ignore certain HTML validity errors

We use the ugc (user generated content) value for no-follow links since #2855
this is not yet supported in the validator.

This patch introduces a list of ignored

Ignore certain HTML validity errors

We use the ugc (user generated content) value for no-follow links since #2855
this is not yet supported in the validator.

This patch introduces a list of ignored error messages.

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added group internet to html validity checks

Because we use an online validator

some basic framework to do at least some HTML validity checking

Checks are done against the w3.org validator API. Currently we have some
fails. One will be fixed in #2314, others still need to be fi

some basic framework to do at least some HTML validity checking

Checks are done against the w3.org validator API. Currently we have some
fails. One will be fixed in #2314, others still need to be fixed.

This integration test also reveals some code bugs because texts do not
suppress warnings and deprecation messages.

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