WTP HTML validator marks any custom html tags as unknown. It's possible to disable this validation at all: Window->Preferences->Web->HTML Files->Validation ==> Set Elements -> Unknown tag name severity to "Ignore":

But it would be useful to have ability to make HTML validator ignore some particular tags only.

I suggest to contribute a patch to WTP to fix it.
We have already did it for attributes, so lets do it for tags too:

• New preferences with a list of ignored tags (a mask can be used)
• A new preference page to set these preferences
• QuickFix for validation problem which will add the tag name/mask to this list/preferences
• Extension in our angular/ionic plugin which will disable validation for <ion-*> tags.

I've filed it to bugzilla - https://bugs.eclipse.org/bugs/show_bug.cgi?id=444545 but we have to provide a patch for it if we want to have it done