You should be aware that the devices are not really emulated. If you have a specific bug on a device, you won't be able to identify it with Chrome Dev Tools. Read more in the google docs. I recommend remote debugging with a real device (for me it is sometimes the only way to reproduce bug reports)! Or using something as browser stack (which allows testing on ...
There are many highlighter extensions available on the Chrome Web Store. Just to name a few:
Weava: unlimited highlights with cloud sync for free
Snippet: free unlimited highlighting with notes; highlights are copied into a Google Keep–like interface