One really useful HTML element that doesn’t seem to get much love is
optgroup. It allows you to group the items of a select list in a way that may be more meaningful (and more readable) to your users than a long, uninterrupted list. The option group labels themselves can’t be selected, so there’s no need for back end logic to filter them out of your form data.
Drupal’s Form API provides option groups, but it isn’t immediately obvious how to use them. A quick check of the select type and #options attribute in the Form API reference doesn’t provide any clues about option groups, so it’s necessary to dive into Drupal’s source code.