This documentation is not using the current rendering mechanism and will be deleted by December 31st, 2020. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
For details have a look at the Github commits: https://github.com/Gernott/mask/commits/master
Take care about this on upgrading Mask:
Upgrade from Mask 3.1.0 or lower: rename all contentelement previewimages from ce_key.png to key.png