I am confused... all the pages start with
<!DOCTYPE html>
<html lang="en">
<head>
<title>...
When I view the source in Firefox, the first line is in italic. Is it commented by FF (last mac version 98.0.1)?
In the web tools console, FF sometimes (not for all the pages) displays a warning saying that the page is in quirk mode and may affect the rendering.
I have read a lot on the subject but I don't seem to understand what I can do to fix this.
Help would be appreciated.
Is it possible for you to inject an iframe without the doctype or for some third party tracking pixel to do it?
If that's the case, I'm afraid your problem is caused by that, you have an answer to the reason here: Dynamic iframe inserted into document that is standards mode defaults to quirks mode