title: Vue.js version 2 ignore customElement that extends form using is= syntax
body: <p>Vue.js version 2 is changing the original syntax of my native customElement and because of the change the browser will not render it on the page.</p>
  <p>Expected html on the DOM <code>&lt;form is=&quot;esm-justification-form&quot;&gt;&lt;/form&gt;</code></p>
  <p>Actual html on the DOM <code>&lt;esm-justification-form&gt;&lt;/esm-justification-form&gt;</code></p>
  <p>In my vue template:</p>
  <pre><code>&lt;div v-if=&quot;isEsm&quot;&gt;
    &lt;form is=&quot;esm-justification-form&quot;&gt;&lt;/form&gt;
  &lt;/div&gt;
  </code></pre>
  <p>In my main app file, I have: <code>Vue.config.ignoredElements = [/^esm-.*/]</code></p>
  <p>In the browser where I register the component <code>defineElement('esm-justification-form', justificationEle, 'form')</code></p>
  <p>I've tested the regex expression outside of the app and it appears to be valid.</p>
  <p>I could change from the 'is' syntax, but then 'this' would no longer be the form element. I have quite a few forms. I do not want to add extra code to select the form inside the component for events and such. There should be a way to prevent this.</p>