You can see the word "Document Count" in Chrome DevTools.
What is it?
For example, recording the below website, the number of documents is 8 or 9.
https://developers.google.com/chrome-developer-tools/docs/javascript-memory-profiling?hl=it#dominators 
This is a question of a tool but I think this could be a question of HTML, so I'm asking here.
The "Document Count" maintains a count of the number of documents used by your application over time, that are held in memory (not garbage collected). Iframes on your page along with their documents are also counted here. This counter may contribute in helping you identify what events in your application may be leading to a memory leak.
Please see the link below: https://developers.google.com/chrome-developer-tools/docs/timeline
来源：https://stackoverflow.com/questions/23214539/what-does-document-count-in-the-devtools-refer-to