Isn't this approach making html more heavy or it doesn't matter?
I am always confused when I should create a new class for an element. Isn't it bad for structure to have many classes? Is it affecting SEO somehow? Where can I read about it?
Joachim Larsen9,280 Points
The CSS classes have no effect on SEO. Google considers the structure of your tags and content, page navigation, URL structure, load times etc.
I think Google's Style Guide on Github is a good resource for reading about structuring HTML and CSS: https://google.github.io/styleguide/htmlcssguide.html