For full detail:
If your Itch page has a “Run Game” button, then IFDB can have a “Play Online” link which just links to the Itch page, where players can click on Itch’s “Run Game” button.
But if your Itch page doesn’t have its own “Run Game” button, then no one but the author can make a web-playable version at all.
Normally, when someone just uploads a .gblorb/.z5 somewhere, IFDB can use the Parchment site, iplayif.com, with an “embedded permalink” to the file as a direct download.
- For example, for Counterfeit Monkey, the
.gblorbis on Github at https://github.com/i7/counterfeit-monkey/releases/download/r11.1/CounterfeitMonkey-11.gblorb - … and IFDB links to https://iplayif.com/?story=https%3A%2F%2Fgithub.com%2Fi7%2Fcounterfeit-monkey%2Freleases%2Fdownload%2Fr11.1%2FCounterfeitMonkey-11.gblorb to make a “Play Online” button.
But Itch doesn’t allow “permalinking,” where there’s a simple link that people can use to download your game files with one click. (When you download a game from Itch, it pops open that little “pay what you want” popup, and then gives you a list of temporary download links; those links only work for you, and only for a limited time.)
So, there’s no way for ordinary users (or IFDB) to construct an iplayif.com link pointing to a file on Itch.
@Denk has documented how an author can use Parchment to make an Itch playable “Run Game” button for anything Parchment supports, but the key critical step is that you have to also upload your .gblorb/.z5 file somewhere else, not Itch, e.g. Google Drive.
It is necessary that your game is available somewhere on the net unzipped unless it is in the IF Archive, then it can work zipped as well.
… and then you have to always remember to post updates to your game twice, once to Itch, and then again to the other host.
This is one reason why it’s valuable for competitions to submit all of their entries to IF Archive right as the competition starts. That way, IFDB can link to IF Archive and create a “Play Online” button there, even if the author doesn’t do (or understand) any of what I said above.