While DataCamp's browser version does not have an offline mode, you can download videos and slides.

Downloading videos:

Click on the download button to the right of the video timer. Your video will then be prepared for download before being saved to your local device.

Downloading slides:

Firstly, click on the page icon in the top right-hand corner. 

Once you have clicked the download button, you will be greeted with the pop-up below.

You can then click the download button, and the files will be saved to your local device!