SteamVR has a hidden configuration parameter which allows to enable supersampling for VR applications. With supersampling graphics are rendered in a higher resolution than the native one of the screen in the headset. Afterwards the image is down-sampled and antialiased. This generates a sharper image because – for example – small lines and other details have to be rendered completely before antialiasing. The downside is that this really raises the burden on the graphics card. Additionally, not all VR games support supersampling. Valve probably had a good reason to keep this configuration parameter hidden, so setting it manually might have negative consequences.
To enable supersampling in SteamVR, one has to edit a JSON formatted configuration file. It's a good idea to create a backup before doing so.