Bro support has been added to PrismJS (which is used in Google Polymer). You can try it out at http://prismjs.com/test.html . If you click the circles on the right side of the page, you can see it rendered in different styles. (Copy-paste some Bro code into the box and select “Bro” as the language.)
Also, there’s an Ace online editor with (limited) Bro syntax highlighting. I created it by importing an existing Bro Textmate file. It’s at https://github.com/open-nsm/ace . It could probably use some manual tweaking (which may happen at some point in the future).