Toggle CSS classes when an element is clicked
There are a lot of votes for adding Javascript support and several for adding more sophisticated animation support.
If there was a way to simply toggle a CSS class with a select number of events (hover, tap, click, keydown) that would go a long way to satisfying these users. By providing the ability to listen for an event and then toggle a class on an element, you can add custom keyframe animations for example or even simply hide or show elements with the display or visibility properties. That in and of itself would be and incredibly powerful (and really safe) way to allow people to create more complex presentations that included quizzes, custom animations, etc.