@Override
  public void registerReferenceProviders(@NotNull PsiReferenceRegistrar registrar) {
    HtmlReferenceProvider provider = new HtmlReferenceProvider();
    String[] htmlAttrs = HtmlReferenceProvider.getAttributeValues();
    ElementFilter htmlFilter = HtmlReferenceProvider.getFilter();
    XmlUtil.registerXmlAttributeValueReferenceProvider(registrar, htmlAttrs, htmlFilter, false, provider);

    XmlUtil.registerXmlAttributeValueReferenceProvider(registrar, new String[] {"content"}, new ScopeFilter(
      new ParentElementFilter(
        new AndFilter(XmlTagFilter.INSTANCE, new XmlTextFilter("meta")), 2
      )
    ), true, new HtmlHttpEquivEncodingReferenceProvider());

  }
@Override
  public void registerReferenceProviders(@NotNull PsiReferenceRegistrar registrar)