Machine learning for predicting HTML Elements on a web page?
There are at least 2 distinct problems:


Understanding the non-programmers intention for the webpage's content 
Understanding the webpage's content


The 1st problem is a problem not best solved with data science or machine learning, hard-coded rules will work better. The 2nd problem is mostly a software engineering problem. I would look into existing solutions for inspiration, for example scraper or import.io.

If you still want to sort and present the "selectors" to users, that is most frequently called a "learning to rank" problem. Learning to rank problems are common in information retrieval systems / search engines.