The following will work for a lot of pages but especially for D.Tube on the most common browsers like Google Chrome, Firefox and also on browsers on mobile devices

On the computer...

only do a right click on the video.
A context menu opens.
Click save video and you are good to go!

On the mobile device...

... start the video and long press on the video.
A menu opens.
Just click "Save video" and you are done.

Have fun :D