Reduce forced layout reflows in init or methods

[dmethvin]

This is a meta-ticket for identifying and solving issues with feature detects or other code that may be forcing layout and impacting performance.

Some of the issues may be resolved via documentation.

[dmethvin]

See comment 52 in #5087, checking for the className before removing it saves a reflow if the class isn't there.

[Jeremy Dunck]

Fix #14164: assign className in addClass/removeClass only if changed. Close gh-1331.

Changeset: c418b94eb48188cd9329519ae5e030a52dd81cc9

[m_gol]

Crap, wrong ticket.

[dmethvin]

Correct ticket for that changeset is #14250

[m_gol]

Is it still relevant after support refactor?

[dmethvin]

It's blocked by an open ticket, but it is probably better to handle the others on a case-by-case basis rather than a meta-ticket.