JSFiddle - React, Tailwind, and code Playground
HTML
<script>
function loadfn () {
document.querySelector('#btnsubmit').disabled = false;
}
</script>
<iframe width='100%' height='300px' src='//jp.wikipedia.org/' onload='loadfn()'></iframe>
<input id="btnsubmit" type="submit" value="test" disabled />