Web updates

HTML 92495928eda2

Simplify getContext() and make it throw less In practice getContext() only has two arguments. And in practice some handling of the second argument is shared across the various rendering context types. This aligns the standard with that practice. This also obviates the need for dedicated coercion algorithms for "2d" and "bitmaprender". This change also fixes various minor nits found along the way, such as the inconsistent spelling of contextId (was sometimes contextType). Tests: and Fixes #595.

Anne van Kesteren