It's not a question of HTML's syntax.
1. HTML is a widely implemented standard. What you learn and know about HTML, and what you create with HTML is widely applicable. Not so much for your HTML alternatives, like Aberdeen.
2. HTML is what the browser accepts, which means you end up dealing with it anyway, just with a transformation in between, making things harder. The bigger the transformation the harder it is. To develop with Aberdeen you still need to know HTML, but you also need to know Aberdeen and how it transforms into HTML. And, you typically end up needing to learn how to mentally transform backwards as you debug, e.g., using the browser dev tools, looking at problem HTML, and deciding how to change the Aberdeen code to fix it.