How to Extend HTML Intellisense in Visual Studio 2017?

Hi

We'd like to extend the intellisense completion list for certain HTML attributes. 

For example, for the "data-foo" attribute we'd like the following values "bar1 | bar2 | bar3".

How do I do this?

I've had a look at the schema files here:

C:\Program Files (x86)\Microsoft Visual Studio\2017\Community\Common7\Packages\Schemas\html

And tried updating the html_5.xsd and the common HTML5Types.xsd. Although I restarted Visual Studio after doing this, it doesn't seem to work.

Any help much appreciated.

Thanks!

Noel