HTML5 is supported in all modern browsers (Chrome 40 and above; Firefox 35 and above; Internet Explorer 10 and above (including Edge); Opera 20 and above; Safari 7.0 and above)
Internet Explorer 9 and earlier does NOT support styling of unknown elements so a backup image will be served
Browser Support Matrix: http://html5test.com/results/desktop.html
If the browser is unable to render the ad, a backup image should be served. Yahoo served ads will serve a backup image to the following browser versions (exceptions may apply):
Chrome 13 and below
Opera 11 and below
Safari 5 and below
Firefox 5 and below
Internet Explorer 9 and below
“Graceful Fail” is recommended if a feature is not available. For example, if the geographic location is not available, the ad can fall back to providing text box for the user to enter the Postal Code