I realize its sacrilegious to say anything bad about HTML5, but I have to say I am really disappointed in its handling of widgets (i.e. combo boxes, lists, sliders, etc.) HTML5 introduces new form input types such as range, date pickers, color selector, etc. but they are completely opaque and have very narrow APIs. This is due to HTML5's approach of punting to the browser to render and manage the widget. Often the browser in turn punts to the OS. This was the approach Java AWT took when it was introduced in 1995 and we all remember how well that went :-/ There is a reason it was nicknamed the Awful Windowing Toolkit!
In HTML5 there is, for example, no way to extend a range to provide two nobs for a price range. You also cannot style it with CSS aside from doing things like putting a border around the outside. If you want to create a custom widget you are left to start from scratch and do all your own painting and event handling with a canvas component or SVG elements. This is a non-trivial endeavor for all but the simplest components. Every other modern RIA platform (Flex, Silverlight, JavaFX, etc.) provides a nice OO library of extendable and skinnable components. What not HTML5?