javascript – How does Firefox reader view operate-ThrowExceptions

Exception or error:


I am looking for the criteria by which I can create a webpage and be [fairly] sure it will appear in the Firefox Reader
, if user desired.

Some sites have this option, some do not. Some with more text do not have this option than others with much less text. Stack Overflow for
instance displays only the question rather than any answers in Reader


I have had my Firefox upgraded from 38.0.1 to 38.0.5 and have found a new feature called ReaderView – which is a sort of overlay which removes “page clutter” and makes text easier to read.
Readerview is found in the right hand side of the address bar as a clickable icon on certain pages.

This is fine, but from the programming point of view I want to know how “reader view” works, which criteria of which pages it applies to. I have done some exploration of the Mozilla Firefox website with no clear answers (sod all programming answers of any sort I found), I have of course Googled / Binged this and this only came back with references to Firefox addons – this is not an addon but a staple part of the new Firefox version.

I made an assumption that readerview used HTML5 and would extract <article> contents but this is not the case as it works on Wikipedia which does not appear to use <article> or similar HTML5 tags, instead the readview extracts certain <div>s and displays them alone. This feature works on some HTML5 pages – such as wikipedia – but then not others.

If anyone has any ideas how Firefox ReaderView actually operates and how this operation can be used by website developers, can you share? Or if you can find where this information can be located, can you point me in the right direction – as I have not been able to find this.

How to solve:

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:

 123456789 123456

See my example at


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 ( ) 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!!

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.


I followed Martin’s link to the Readability.js GitHub repository, and had a look at the source code. Here’s what I make of it.

The algorithm works with paragraph tags. First of all, it tries to identify parts of the page which are definitely not content – like forms and so on – and removes them. Then it goes through the paragraph nodes on the page and assigns a score based on content-richness: it gives them points for things like number of commas, length of content, etc. Notice that a paragraph with fewer than 25 characters is immediately discarded.

Scores then “bubble up” the DOM tree: each paragraph will add part of it’s score to all of it’s parent nodes – a direct parent gets the full score added to its total, a grandparent only half, a great-grandparent a third and so on. This allows the algorithm to identify higher-level elements which are likely to be the main content section.

Though this is just Firefox’s algorithm, my guess is if it works well for Firefox, it’ll work well for other browsers too.

In order for these Reader View algorithms to work for your website, you want them to correctly identify the content-heavy sections of your page. This means you want the more content-heavy nodes on your page to get high scores in the algorithm.

So here are some rules of thumb to improve the quality of the page in the eyes of these algorithms:

  1. Use paragraph tags in your content! Many people tend to overlook
    them in favor of <br /> tags. While it may look similar, many
    content-related algorithms (not only Reader View ones) rely heavily
    on them.
  2. Use HTML5 semantic elements in your markup, like <article>, <nav>,
    <section>, <aside>. Even though they’re not the only criterion (as you noted in the question), these are very useful to computers reading your
    page (not just Reader View) to distinguish different sections of
    your content. Readability.js uses them to guess which nodes are likely or unlikely to contain important content.
  3. Wrap your main content in one container, like an <article> or <div>
    element. This will receive score points from all the paragraph tags
    inside it, and be identified as the main content section.
  4. Keep your DOM tree shallow in content-dense areas. If you have a lot
    of elements breaking your content up, you’re only making life harder
    for the algorithm: there won’t be a single element that stands out
    as being parent of a lot of content-heavy paragraphs, but many
    separate ones with low scores.

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