Considering many HTML5 features and related JavaScript APIs can easily be made to work in older browsers using polyfills, many relatively larger sites have been doing this for lots of things (HTML5 block elements in IE<9, for example).
Considering many HTML5 features and related JavaScript APIs can easily be made to work in older browsers using polyfills, many relatively larger sites have been doing this for lots of things (HTML5 block elements in IE<9, for example).