Question: It's a common practice to highlight an element on hover state. 
By highlighting it, I obviously mean making it more visible than other elements.

But lately, I've witnessed the opposite trend on a couple of websites : 
Opacity is set to 0.5 or less on hover, which makes the hovered element less visible instead.

To me, this is nothing but a bad practice.

Or is it a new trend backed by some well known data that I'm unaware of?

Answer: 
### Which of these things is not like the other?

[I tried lowering the opacity to 50%](http://codepen.io/run-time/full/MYPZxM/) to see how it looked and without the context of the mouse cursor **this could be interpreted as a disabled button**.

![enter image description here](https://i.stack.imgur.com/eJv08.png)

With the context of the mouse cursor, however, I really don't think users would get confused.

On the other hand, highlighting the button feels more inviting and has **less of a chance that a user interprets the button as unavailable** (even without the cursor)

![enter image description here](https://i.stack.imgur.com/mW2Ng.png)
I agree that it is **currently** a bad practice, primarily because it is returning to the days of metaphoric design. The designer is trying to inspire the appearance of (and a realworld reference to) a frosted sheet of glass floating above the surface of the rest of the interface.

In this age of Modern-UI, it is common practice to avoid metaphoric/skeuomorphic elements in our designs.

From a purely UI standpoint, I don't think there is anything wrong with floating sheets of frosted glass. They are pretty in a nostogic kind of way... and they do focus the user's attention on a subset of input options which is what they are supposed to do. 

They just are currently not popular; non-standard enough to be considered mainstream yet still too familiar to called revolutionary. Just a little too... dated.