Steam may be the best digital gaming platform for PCs, but it sure is not without its flaws. One of the biggest disadvantages is that you can't activate keys from a web browser, you need the Steam client to do that.
So, when you buy a game from a third party website, or a bunch of games, say from Humble Bundle, you had to copy the key, switch to the Steam application, click a button and paste the key to redeem the game. This can be a tedious task, if you have to activate multiple games at once. More importantly, if you didn’t have access to the Steam client on the computer you are using, or are away from your PC (at work, or vacation, etc), you cannot activate the games.
But there is good news, as Steam now allows activation of keys from web browser. There has not been an official announcement about it yet, but the process works.
How to activate a Steam key from a browser:
1. Visit the following page from your desktop web browser. (you can bookmark it for your convenience).
2. You will be asked to login to the website (and authenticate the login via the mobile app's Steam Guard).
3. This will bring the "Activate a Product on Steam" webpage. You can paste the product code here, agree to the terms, and click on the continue button.
4. This will add the game to your Steam Library.
According to some redditors, you can also add the key to the URL like this: (though we think this is a tedious task to add the extra letters).
This feature is not available in the mobile app.