Remove non-standard tap-highlight-color and touch-callout CSS properties

Problem/Motivation

Originally found in #3331424: Drop -moz- vendor prefixed properties and then discussed in Slack with @alexpott, @mherchel and @andy-blum.

In toolbar's CSS we have the following non-standard properties:

  -moz-tap-highlight-color: rgba(0, 0, 0, 0);
  -o-tap-highlight-color: rgba(0, 0, 0, 0);
  -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
  tap-highlight-color: rgba(0, 0, 0, 0);
  -moz-touch-callout: none;
  -o-touch-callout: none;
  -webkit-touch-callout: none;
  touch-callout: none;

These were originally introduced in an early prototype of the toolbar CSS in #1137920: Fix toolbar on small screen sizes and redesign toolbar for desktop

These properties are currently only supported by Safari on iOS, with the -webkit- vendor prefix, and even then we are not sure that they do anything useful.

https://developer.mozilla.org/en-US/docs/Web/CSS/-webkit-touch-callout
https://developer.mozilla.org/en-US/docs/Web/CSS/-webkit-tap-highlight-c...

Steps to reproduce

Proposed resolution

Remove these properties.
Check for regressions on iOS.
If the properties are useful:

• Keep only the -webkit- prefixed versions.
• Add a comment explaining what the properties do.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet