Just updated the Opera Mini version on my phone, then saw the feature:
You can now select text on a web page via two sliders and a select button. You can drag the sliders to indicate the text range you want to select.
The sliders are transparent (sort of), so you can see the bits of text under them, which helps in making your selection (before you copy / paste it).
I guess they are using something like jQuery Mobile or an equivalent library (or rolled their own) to achieve this.
The feature is a bit buggy as of now, though, at least on my Android phone.