From this post on MSDN Blogs:
The reading view button will appear in the address bar for pages that have “article-like” content. If you don’t see it, it means that the page doesn’t work well in reading view.
That's rather vague and unhelpful. Luckily, our friends at Stack Overflow have managed to narrow it down a bit:
- You have to have a non-empty
<title> element. It doesn't matter what's in it as long as there is at least one non-whitespace character.
- You also have to have a non-empty
<h1> element. This will be used as the page title in the reading view. It doesn't have to match the
- After that, you need at least 700 characters of content.
The main requirement seems to be the 700 characters of content. Note that what exactly qualifies as "content" is not entirely clear; it certainly doesn't include all text on a webpage.
(While the above links are about IE 11 on Windows 8.1, the information should apply equally to IE 11 in Windows Phone 8.1 as well.)