Since clientPrefs and other related features are not avaliable until MW 1.42,
many of them are backported as polyfill. Instead of using cookies, the polyfill
are using localStorage only like Citizen in the past.
There are many changes behind the scene, but the most important one being that
`skin-citizen-*` theme classes are now soft-deprecated, and replaced by the
standardized `skin-theme-clientpref-*` classes. There will be sufficient
time before the hard deprecation.
Related: #780
* Adjust font size slider range, it starts at 80% and ends at 180%
* Font size is always shown in percentage now
* Better handling of initial font size, it should now detect browser or site default correctly
* Use relative units
* Tweak styles
