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You can copy-paste a word from the website into your search engine too. The website doesn't have to do anything. Just like the program you used on your iPad didn't do anything, it was the iPad providing the select-and-dictionary feature.

Sounds great as a feature of your client, not of every single site.

On the Mac too, you can just "force press" the touchpad on any word and a definition comes up. As a non-native English speaker, I use it quite a lot.

or highlight and ctrl+cmd+d :)

Firefox has an option for this in the right-click menu after selecting text (e.g. by double-clicking).