To support roll out and avoid issues with cached HTML the new
styles for the new search feature are restricted to HTML where
the body tag has `skin-vector-search-header` class.
For legacy mode, we introduce a new class
`skin-vector-search-header-legacy` and temporarily use a CSS3 `:not()`
selector to ensure the styles ship during the phase where cached
HTML can be served. While this will create some display issues in
browsers that do not support CSS3 selectors, all grade A browsers in
our compatability matrix support this.
Bug: T249363
Change-Id: I7f8059d43eaab49de362405784b34a4fe502c7b0
