How does Safari's Reader work and when does it show up?
Safari's Reader makes use of Readability.
Readability turns any web page into a clean view for reading now or later on your computer, smartphone, or tablet.
There's this nice article about how Safari's Reader detects the text, you should consider reading.
It looks like that Safari Reader will detect a
<div>or block level element that contains a header element (<h1>to<h6>), followed by a certain amount of text. The reader badge will appear when the content text (not including the header) is more than 2,000 characters.
There's a Readability extension for Chrome which you can get from here.
You can take a look at its source code
Method how to obtain it is here
Evernote offers a similar plugin for Chrome called Clearly that also integrates with an Evernote Notebook if you use that system.
https://evernote.com/clearly/
There is also a Clearly plugin for Firefox.
Installing the plugin gives you a toolbar button next to the address bar that you click to view a "clean" version of the page and which includes a few GUI tweaks for low light or background color change if beige offends your sensibilities, as well as a button to copy the cleared page to an Evernote Notebook.