Why is <marquee> deprecated and what is the best alternative?

Longer time I'm curious about HTML tag <marquee>.

You can find in MDN specification:

Obsolete This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.

or on W3C wiki:

No, really. don't use it.

I searched several articles and found some mention about CSS relevant replacement. CSS attributes like:

marquee-play-count
marquee-direction
marquee-speed

but it seems, they don't work. They were a part of specification in year 2008, but they were excluded in year 2014

One way, proposed by W3 Consortium, is using CSS3 animations, but it seems for me much more complicated than easy-to-maintain <marquee>.

There are also plenty of JS alternatives, with lots of source code that you can add to your projects and make them larger.

I'm always reading things as: "don't ever use marquee", "is obsolete". And I don't get why.

So, can anybody explain to me, why is marquee deprecated, why is so "dangerous" using it and what is the easiest substitution?

I found an example, it looks nice. When you use all prefixes needed for good browser support, you have around 20-25 lines of CSS, with 2 values hardcoded (start and stop indent), depending on text length. This solution is not so flexible, and you can't create bottom-to-top effect with this.

I don't think you should move the content but that doesn't answer your question... Take a look at the CSS:

.marquee {
    width: 450px;
    line-height: 50px;
    background-color: red;
    color: white;
    white-space: nowrap;
    overflow: hidden;
    box-sizing: border-box;
}
.marquee p {
    display: inline-block;
    padding-left: 100%;
    animation: marquee 15s linear infinite;
}
@keyframes marquee {
    0%   { transform: translate(0, 0); }
    100% { transform: translate(-100%, 0); }
}

Here is the codepen.

Edit:

Here is the bottom to top codepen.

You just have to define class and attached looping animation once in CSS and use it afterwards everywhere you need. But, as many people said - it's a bit annoying practice, and there is reasone, why this tag is becoming obsolete.

.example1 {
  height: 50px;	
  overflow: hidden;
  position: relative;
}
.example1 h3 {
    position: absolute;
    width: 100%;
    height: 100%;
    margin: 0;
    line-height: 50px;
    text-align: center;

    /* Starting position */
       -moz-transform:translateX(100%);
       -webkit-transform:translateX(100%);	
       transform:translateX(100%);

 /* Apply animation to this element */	
       -moz-animation: example1 15s linear infinite;
       -webkit-animation: example1 15s linear infinite;
       animation: example1 15s linear infinite;
}

/* Move it (define the animation) */
      @-moz-keyframes example1 {
       0%   { -moz-transform: translateX(100%); }
       100% { -moz-transform: translateX(-100%); }
      }
      @-webkit-keyframes example1 {
       0%   { -webkit-transform: translateX(100%); }
       100% { -webkit-transform: translateX(-100%); }
      }
      @keyframes example1 {
       0%   { 
       -moz-transform: translateX(100%); /* Firefox bug fix */
       -webkit-transform: translateX(100%); /* Firefox bug fix */
       transform: translateX(100%); 		
       }
       100% { 
       -moz-transform: translateX(-100%); /* Firefox bug fix */
       -webkit-transform: translateX(-100%); /* Firefox bug fix */
       transform: translateX(-100%); 
       }
      }
    

<div class="example1">
   <h3>Scrolling text... </h3>
</div>

<marquee> was never part of any HTML specification and what you link to is a CSS spec so it's hard to deprecate something that was never included. HTML is about structure of a document, not its presentation. So having a self-animated element as part of HTML does not abide by those goals. Animation is in CSS.

As stated before: the easiest substitution is CSS animation

To all the critics of the marquee:

It is a very useful tool for UI, I am using it just on hover, to display more information in a limited space.

The example for the mp3-player is excellent, even my car-radio is using the effect to show the current song.

So nothing wrong about that, my opinion ...