I write an application and inside of HTML code I have custom tags (of course these tags are parsed on server side and end user gets them as valid HTML code). Example of custom tag usage:
<Gallery type="grid" title="My Gallery" />
Sadly, there is no easy way: you have to roll your own parser for this, and then add both your extra elements and the base grammar (HTML) to it.
If you have your parser, you could use it to do syntax highlighting (strictly speaking, for that simple lexing is enough); and a good parser can support content assist (auto-suggestions in your terminology).
Some help on the other hand: you could use some text editor generators to ease your work: