Here are a few HTML5 features that developers may not be know of:

In HTML, the 'dialogue' element is used to generate native modal dialogue windows.

The 'picture' element is used to specify various pictures for various devices and screen sizes.

The 'contextmenu' property in HTML5 allows you to create custom context menus.

HTML5 adds new form components such as 'date', 'time', 'range', and 'search'.