Before calling Panel:SetHTML or DHTML:OpenURL, the result seems to be false with the Awesomium web renderer and true for the Chromium web renderer. This difference can be used to determine the available HTML5 capabilities.

On Awesomium, the result remains true until the root document is loaded and when in-page content is loading (when adding pictures, frames, etc.). During this state, the HTML texture is not refreshed and the panel is not painted (it becomes invisible).

On Chromium, the value is only true when the root document is not ready. The rendering is not suspended when in-page elements are loading.