How does Firefox reader view operate
Solution 1:
You need at least one <p> tag around the text, that you want to see in Reader View, and at least 516 characters in 7 words inside the text.
for example this will trigger the ReaderView:
<body>
<p>
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789 123456
</p>
</body>
See my example at https://stackoverflow.com/a/30750212/1069083
Solution 2:
Reading through the gitHub code, this morning, the process is that page elements are listed in a likelyhood order - with <section>,<p>,<div>,<article> at the top of the list (ie most likely).
Then each of these "nodes" is given a score based on things such as comma counts and class names that apply to the node. This is a somewhat multi-faceted process where scores are added for text chunks but also scores are seemingly reduced for invalid parts or syntax. Scores in sub-parts of "node" are reflected in the score of the node as a whole. ie the parent element contains the scores of all lower elements, I think.
This score value decides if the HTML page can be "page viewed" in Firefox.
I am not absolutely clear if the score value is set by Firefox or by the readability function.
Javascript is really not my strong point,and I think someone else should check over the link provided by Richard ( https://github.com/mozilla/readability ) and see if they can provide a more thorough answer.
What I did not see but expected to see was score based on amount of text content in a <p> or a <div> (or other) relevant tags.
Any improvements on this question or answer, please share!!
EDIT:
Images in <div> or <figure> tags (HTML5) within the <p> element appear to be retained in the Reader View when the page text content is valid.