Yes, it depends whether the Readability code can identify the content. This varies somewhat depending on the BB markup and the post itself:
How to enable Safari Reader on your site? · Mathias Bynens
It turns out Readability then loops through all these ‘paragraphs’ and assigns a score to them based on how ‘content-y’ they look. This score is determined by things like the number of commas, class names used in the markup, etc. The content’s length appears to be measured by using .innerText; for every 100 characters inside a paragraph, that paragraph’s score goes up. Eventually, the number of elements is counted, adding their individual scores. I think it’s safe to assume Safari Reader is triggered based on this algorithm.
Video-only posts may never work since there’s basically no text content to work with.
We might have better luck lobbying BB to standardize markup a bit more.
