itch.io

The tagline is expanded as a tooltip by hovering over it. But I guess one would not do that, if the first half is not peaking interest. Also, if someone hovers over the game, then probably over the cover and not over the tagline. So there will be a bigger thing. The popup with the screenshots and the cutted off first paragraph of the game description.

Steam uses an actual blurb. That's not a tagline on Steam descriptions. It is always displayed in full. Also, you see them only if you are already on the complete game description page. You do not see the taglines/blurbs while browsing for games on Steam. Not even in a popup. There are popups, but they show screenshots. (At least I did not find a place where they show the tagline.)

Interestingly enough, the tagline is not shown on the project page. Which is kinda lame.

You cannot influence where the tagline is cut off. That is dependant on the user's interface. It has about as much width as the image over it. And that changes, if you change zoom on the browser. The 36 item grid also changes, so you can have 3x12 or 4x9 or 6x6. And with changes in size the browser might chose a different rendering of the letters. And I did not even look how this is shown on mobile or in the Itch app.

So what to use the "tagline" for? Anything that fits. Literally and figuratively. You have little space, but you can enter information there that complements the picture or title. I see many projects that give information in the title. That's bad for searches. They could shift that information in the tagline. Or if you have a generic cover and title, you can add interesting bits to make it more appealing.

From a platform design point of view, I would have assumed that the tagline/short descripion/blurb would show up in the popup along with the screenshots. Since it is not shown in the game description, that feels like a wasted opportunity. And the first paragraph of game descriptions often contains not really helpful things to make the game appealing.