Only for JS developer when they have to answer some side questions to make interviewer comfortable...
HTML might be heart of a website, but least utilized and explored. js dude followed the same path and put some basic questions, when you are applying for position directly not for html/markup developer or designer.
getting high level.
Browser is your play ground. The better you know your play ground, the easier for you to play the game. and sometimes you can easily play the trick. Some core browser concepts will make you a good developer. Hence interviewer wants to make sure, you have enough knowledge to win in the battle field..
Thats what you are, a web development, a plain simple web developer. you have to know the core of web development technologies. A way to optimize user experience, resource management and overall architecture of the internet. Otherwise you will fail miserably in the larger scale of website.