Tests: Re-add CSS hack to make the test pages look OK in a browser

The hack was removed in d14b90ded2f3e3845f001d15047f4a4d4fad3840.

Fixes gh-7025
Slider: Fix CSS structure special comment reference

Fixes gh-7027
Slider: Initially hide the tooltip

Fixes gh-7020
Toolbar: Spelling correction in persistent toolbar demos