This color effect is pure CSS tagged inside html with the < style > tag.

The following CSS creates this effect.

     <style>
             
   .wrapper {<--- color.JesseJesse.com --->
    height: 100%;
    width: 100%;
    left:0;
    right: 0;
    top: 0;
    bottom: 0;
    position: absolute;
    background: linear-gradient(124deg, #ff2400, #e81d1d, blue, #e3e81d, #1de840, #1ddde8, red, #dd00f3, #dd00f3);
    background-size: 1800% 1800%;
    -webkit-animation: rainbow 18s ease infinite;
    -z-animation: rainbow 18s ease infinite;
    -o-animation: rainbow 18s ease infinite;
    animation: rainbow 18s ease infinite;}
    @-webkit-keyframes rainbow {
       0%{background-position:0% 82%}
       50%{background-position:100% 19%}
       100%{background-position:0% 82%}
    }
    @-moz-keyframes rainbow {
       0%{background-position:0% 82%}
       50%{background-position:100% 19%}
       100%{background-position:0% 82%}
    }
    @-o-keyframes rainbow {
       0%{background-position:0% 82%}
       50%{background-position:100% 19%}
       100%{background-position:0% 82%}
    }
    @keyframes rainbow {
       0%{background-position:0% 82%}
       50%{background-position:100% 19%}
       100%{background-position:0% 82%}
    }
</style>

Created by Jesse@JesseJesse.com

CSS was released in 1996 and is the language developers use to style an HTML document.

It's common to see images embeded on webpages.