Side note: it just needs a CSS change, as WKWebView already should implement dynamic type. See http://www.interactiveaccessibility.com/blog/text-resizing-web-pages-ios-using-dynamic-type#.WQiJ8ImGORs for a textual version, and there is a WWDC video where this is also discussed. 

wychen: if I understand correctly this change should be done in the CSS of the downloaded page. Do you think it is possible?

There's a pinch-to-zoom feature almost nobody knows. It's hard for users to discover it naturally since zooming is usually disabled on mobile sites. Might make sense to add in-page control like Reader Mode on Firefox.

For dynamic type support, you can hack around in this preview page:
https://cs.chromium.org/chromium/src/components/dom_distiller/core/html/preview.html

Open the file locally on your browser from your checked out repo, and modify its CSS.

The pinch to zoom feature is nice, but I think it is different from the dynamic type.
From what I understand, the idea would be to inject a iOS-specific css defining the fonts specified by apple. It would depend on how many different paragraph type dom distiller is creating.

I gave it a try, and the system fonts looks like serif. We use sans-serif by default. Since we don't expose the theme color/font/size config to the user, they'll see this change and have no way to change it back.

One could use:

p {
  font: -apple-system-body;
  font-family: Arial, sans-serif;
  font-weight: normal;
}

See the original link in Comment #1

Would you mind tweaking the CSS to your liking so that it shows dynamic type on iOS? I guess it doesn't have to look the same as on other platforms. I can help verify that the CSS doesn't cause behavioral changes on other platforms. Would this work? I'm removing myself from the owner.

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Olivier, could you take a look at this reader mode bug?  It looks like we can update the CSS of the distilled page to respect the system a11y text size.