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 these days there are many frameworks provide an ability to create custom tags and attributes, so there cannot be some static schema created in order to help to validate such pages.
We need to add a possibility to provide a custom validator for tags/attributes/values for WTP, so it could be called to validate a page elements.
See discussions here:
JBIDE-18387 and here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=443097.