The theme was initially developed using WordPress 3.8.1. This site is currently running WordPress 4.9.9. There will be no attempt to try and maintain back compatibility with older versions.
The initial conversion and development of the theme used code that would have been perfectly at home in HTML 4.01 Strict, with just a few tweaks to make it HTML 5. In the longer term more HTML 5 specific code might perhaps be introduced, for example some of the instances of <div> may be replaced by a more suitable tag, but it isn't really a priority. The theme uses various bits of HTML that are generated by WordPress itself. These are in HTML 5 format where available and do use HTML 5 specific tags.
The HTML and CSS contained within the theme itself should pass W3C validation and any validation errors will be treated as bugs that need addressing. It is quite likely you will find errors on your site but they should be coming from code generated by WordPress or third party plugins, not by the theme. Similarly any warning or error messages you might receive from PHP should be generated by plugins you are using and not the theme.
The theme is most frequently checked using the latest versions of Microsoft Edge on Windows and Google Chrome on Windows and Android.
The HTML 5 shim script has been installed to help out very old versions of Internet Explorer but it is no longer checked.