You did not find anything, because you cannot send an email with HTML, HTML is merely a markup language and has nothing to do with any sort of processing.
You can use any server-side programming framework to send an email, or even a language such as C++/C# or Java. This topic has been covered enormously, that even writing this post was a redundancy. For example, consider this article,
Universal SMTP code to send emails in .NET apps[
^], this was for C#, there are many other libraries written for Java, C++, Python and rest to send the emails.
You can use JavaScript to do this, but that would still require someone's backend server such as the Gmail REST API to send emails.
Gmail API | Google Developers[
^]