How to provide extra attributes to tags using xsd descriptors on an intellij plugin?

I'm developping a plugin where I register a new xml extension like below:

public class NsHtmlExtension extends HtmlXmlExtension {

    @Override
    public boolean isAvailable(PsiFile file) {
        return super.isAvailable(file) &&
                NsFacade.getInstance().isNsProject(file.getProject());
    }

    @Nullable
    @Override
    public String[][] getNamespacesFromDocument(XmlDocument xmlDocument, boolean b) {
        String[][] strings = {{"", NsResourceProvider.NS_DTD_URI}};
        return strings;
    }

} 

On the html files now, I have the tags autocompletion using xsd descriptors.

I want to add some extra attributes but it seems that tags using xsd descriptors don't ask attribute providers for additional attributes.

How to add extra attributes for this extentions? for example if a tag Label have a "width" attribute I want to add "i:width", "a:width", "i.width" .."a.width" ...

thanks in advance.