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