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