This is a little bookmarklet that removes the very annoying message "The Adobe Flash Player is required for video playback. Get the latest Flash Player". It turns out that mostly this is a lie and simply removing the elements from the DOM of the YouTube reveals the HTML5 Player and you can watch the video.

Remove YouTube "Flash Required"

javascript:(function(){
    derror = document.querySelector('.ytp-error');
    derror.parentNode.removeChild(derror);
    dloader = document.querySelector('.html5-video-loader');
    dloader.parentNode.removeChild(dloader);
}());

// Use the single line version below for your bookmarklet.
javascript:(function(){derror=document.querySelector('.ytp-error');derror.parentNode.removeChild(derror);dloader=document.querySelector('.html5-video-loader');dloader.parentNode.removeChild(dloader);}());