Flash of unstyled content (FOUC) in Firefox only? Is FF slow renderer?

Question

I\'m not seeing this issue in any other browser that I\'ve tested - IE, Chrome, Opera - but whenever I load a page from the server, I\'m seeing a flash of unstyled content befor

Answer 1

In my case the reason of FOUC in FF was the presence of iframe on page. If I removing iframe from markup then FOUC disappears.

But I need iframe for my own hacking reasons so I changed this

<iframe name="hidden-iframe" style="display: none;position:absolute;"></iframe>

into this

<script>

  document.addEventListener('DOMContentLoaded', ()=>{
    let nBody = document.querySelector('body')
    let nIframe = document.createElement('iframe');
    nIframe.setAttribute('name', "hidden-iframe");
    nIframe.style.display = 'none';
    nIframe.style.position = 'absolute';
    nBody.appendChild(nIframe);
  });
</script>

I've added this inline JS right in template just for readability: in my case this code runs once per page. I know that it's dirty hack, so you can add this code in separated JS-file.

The problem was in Firefox Quantum v65.