premultiplyAlpha behavior does not specify overflow handling.

When premultiplyAlpha=true is specified, it is possible for applications to write pixels that have color component values greater than the value of the alpha channel. This may cause color values to overflow when divided by alpha (e.g. when toDataURL is called). The spec does not currently specify how this case should be handled.

The correct behaviour should probably be to clamp.

Review the WebGL specification sections covering premultiplyAlpha and toDataURL, then trace how pixel color components and alpha are described during conversion. Define and document the overflow behavior, including whether clamping is required, and update any relevant specification text or tests identified there.

Written by the indexing model from the issue text.