function assertFirstTagValid(html) {
  if (asserts.ENABLE_ASSERTS) {
    const matches = html.match(INVALID_TAG_TO_RENDER);
    asserts.assert(
        !matches,
        'This template starts with a %s, which ' +
            'cannot be a child of a <div>, as required by soy internals. ' +
            'Consider using goog.soy.renderElement instead.\nTemplate output: %s',
        matches && matches[0], html);
  }
}
Checks that the rendered HTML does not start with an invalid tag that would
likely cause unexpected output from renderAsElement or renderAsFragment.
See {@link http://www.w3.org/TR/html5/semantics.html#semantics} for reference
as to which HTML elements can be parents of each other.
@param {string} html The output of a template.