The reader mode should have the ability to have a line reader. This enables highlight feature on current line being read using the mouse.
See image for more description.
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this enables users to better focus on the current line being read and also enhances the reader mode greatly.
Why is this useful?
Currently most browsers are in the market of providing distracting features to their users. This feature provides enhanced laser focused attention to the task at hand and hence providing great user experience.
This single feature is why I feel iPad reading is a better experience than kindle ebook. That's a matter of opinion but I strongly feel this is a a must have feature.
Hope this gets more eyes and implemented. The feature is a simple css with mouse pointer having a bright CSS and the rest of the page being dimmed.
The chrome extension for reference: https://chromewebstore.google.com/detail/reader-line/fikijclmnnepcpijbljojpdaepikaicm