I've always known that if you press Ctrl-R on a Java variable in the NetBeans Java editor, all other instances of the variable are highlighted, and can be renamed at the same time.
This is called "Instant Rename".
However, today I discovered (simply because I was reusing my Java editor knowledge automatically in the HTML editor) that the same feature exists in the HTML editor:
Here I've pressed Ctrl-R while the cursor was in the first "h2" and then automatically a square appeared around the matching "h2", after which changes made to the first "h2" were replicated in the second. For example, changing an "h2" to an "h3" is now easier than I had thought. Cool!