How to make an animated background in html?

Question

the frames should be constly changing and be repeated in a cycle

Answer 1

What do you call << animated >> background exactly?

Strictly technically, css 'background-image' property isn't animatable, but background-color is ^^
However, you can have << animated background >> by using gif animated files (not a good idea, as animated gif would grow to much quickly in size), or even using a video element standing back your content, or even better using <svg> element (wich are often lighter than bitmaps/videos, if well designed)... as you can have static background just moved to make it << animated >> ;P

Answer 2

You can insert pictures or colour it in Html.
if you want to do animation then you have to learn
Css best for Animation perpose.