Let me be sure I understand the current situation.
Your theme and the radio player work fine with `<! DOCTYPE html>
<html>` but you’ve identified two plugins that fail to work with the same declaration. This is on the initial page load. But if you simply refresh the same page, every thing works fine. Is this all correct?
If so, this is starting to sound like a server configuration problem more than a WordPress problem. Who are you using as a host?
I’m not entirely sure of all the syntax nuances, but maybe you should remove the space after the ! before DOCTYPE and provide a lang
attribute for the <html>
tag, like this:
<!DOCTYPE html>
<html lang="it-IT">
I’m assuming “it-IT” is appropriate for your site, something else may be more appropriate. I doubt any of that will make any difference, but it is more “proper” for declaring a HTML5 compliant site.
You should try validating the site at https://validator.w3.org/#validate_by_uri+with_options . It’ll probably identify a number of issues, many are of small consequence. We’re mainly looking for serious errors.