I've been keeping up with the latest CSS3 elements such as box-shadow and text-shadow:
-moz-box-shadow: 10px 10px 5px #888;
-webkit-box-shadow: 10px 10px 5px #888;
However, in your experience, have you come across any common pitfalls or gotchas (related to HTML5, CSS3, or JavaScript) that others should be aware of? Of course, testing across different browsers is crucial, but it would be helpful to learn from your insights to avoid making these mistakes in the first place.