Add Custom HTML Tags

I was able to make the new tags available for completion, but they are still not showing up as valid input. I thought I'd paste the code if anyone else was looking to do the same. I would still like to add the same elements to the custom tags in the unknown tag inspection.

public class ISMLTagDescriptorsProvider implements XmlElementDescriptorProvider, XmlTagNameProvider {
    private final String[] ismlTagNames = {
            "isactivedatacontext",
            "isactivedatahead",
            "isanalyticsoff",
            "isbreak",
            "iscache",
            "iscomment",
            "iscomponent",
            "iscontent",
            "iscontinue",
            "iscookie",
            "isdecorate",
            "iselse",
            "iselseif",
            "isif",
            "isinclude",
            "isloop",
            "ismodule",
            "isobject",
            "isprint",
            "isredirect",
            "isremove",
            "isreplace",
            "isscript",
            "isselect",
            "isset",
            "isslot",
            "isstatus",
    };


    @Nullable
    @Override
    public XmlElementDescriptor getDescriptor(XmlTag tag) {
        return null;
    }

    @Override
    public void addTagNameVariants(List<LookupElement> elements, XmlTag tag, String prefix) {
        for (String tagName : ismlTagNames) {
            elements.add(LookupElementBuilder.create(tagName));
        }
    }
}

You need to provide implementation of getDescriptor() to make it work. see com.intellij.codeInspection.htmlInspections.HtmlUnknownTagInspectionBase#checkTag