Hello Martin Dusil
Thanks for bringing up the issue on the front. Unfortunately, this cannot be overriden on Helix Ultimate. You have to customize the original file. I have updated the file by replacing <main> tag with <div> tag. But every time you update Helix Ultimate, it will be changed to default <main> tag. So, keep this file separately. I will request the developer team to check this. If possible, then they will update it in the main file in the future to comply with W3C validator.
Please go to this location:
plugins/system/helixultimate/layouts/frontend
and replace conponentarea.php file with the attached file. You need to unzip the file and then replace.
conponentarea.php.zip
Best regards