isContentHandlerRegistered

isContentHandlerRegistered

[1] Use USVString for all URLs ( (domenic著, )) <https://github.com/whatwg/html/commit/018b983b77b2cd908f6d00100e7e0abe893dd2c3>

[2] Remove registerContentHandler() and several friends (annevk著, ) <https://github.com/whatwg/html/commit/b143dbc2d16f3473fcadee377d838070718549d3>