there is a list off all HTML tags, if a tag with that name exists, it will be treated as a TAG, if it doesn't, it will be treated as a BEM block. That simple.

there is a list off all HTML tags, if a tag with that name exists, it will be treated as a TAG, if it doesn't, it will be treated as a BEM block. That simple. This gives some restrictions to Blocks' names, but it's not a big problem, cause normally you won't give block a name which matches existing HTML tag. The same restrictions are for Modifiers (pseudo-selectors) if it exists, then pseudo, else Modifier.