Table of contents

Triggering CSS animations & transitions

Triggering CSS animations relies on adding and removing classes, done via JavaScript. You can achieve the same behavior on AMP pages by using the toggleClass action. With toggleClass, AMP pages can add, remove, and toggle CSS classes like non-AMP pages.

Defining CSS and Keyframes

You can define CSS in AMP in the follow ways:

Within the <style amp-custom> tag inside the head of the document. 50,000 byte limit.
Within the <style amp-keyframes> tag inside the head of the document. 500,000 byte limit. Restricted to keyframe properties.
Inline styles. 25,000 byte limit.

To keep you pages lean and speedy, AMP has enforced a 50,000 byte CSS limit in the <amp style-custom> tag. While you can use this to define animation styles,the 500,000 bye limit inside of <amp style-keyframes> tag allows for more verbose animations that won't take away precious site style resources.

...
  <style amp-custom>
    div {
      width: 100px;
      height: 100px;
      background: red;
      position: relative;
      animation: mymove 5s infinite;
    }
  </style>
</head>
<body>

<div></div>
  <style amp-keyframes>
   @keyframes mymove {
      0%   {transform: translatey(0px);}
      25%  {transform: translatey(200px);}
      75%  {transform: translatey(50px);}
      100% {transform: translatey(100px);}
    }
  </syle>
</body>
...

Adding, Removing, and Toggling Classes

The AMP action, toggleClass enables the addition and removal of classes to defined elements.

elementName.toggleClass(class="className")

You can toggle a class on the same element you'd like users to interact with, such as an animated hamburger menu.

 <div id="hamburger" tabindex=1 role=button on="tap:hamburger.toggleClass(class='close')">

The toggleClass action can apply to other elements as well and toggle between two classes by adding the force attribute.

<button on="tap:magicBox.toggleClass(class='invisible', force=true),magicBox.toggleClass(class='visible', force=false)">
    Disappear
</button>
<button on="tap:magicBox.toggleClass(class='visible', force=true),magicBox.toggleClass(class='invisible', force=false)">
    Reappear
</button>

If you need to remove a class and disallow reapplication, add the force attribute with a value of false. If you need to addd a class and disallow removal, add force with a value of true.

Animate with Numerous Classes

The amp-bind extension works by defining different states as a JSON object inside <amp-state>.

You can define a list of CSS classes within the <style amp-custom> tag in the head of the document:

...
  <style amp-custom>
    div {
      height: 100px;
      width: 100px;
      margin: 1em;
      background-color: green;
      margin-left: 100px;
      transition: 2s;
    }
    .visible {
      opacity: 1;

    }
    .invisible {
      opacity: 0;
    }
    .left {
      transform: translatex(50px)
    }
    .right {
      transform: translatex(-50px)
    }

    button {
      margin-top: 120px;
      margin-left: 1rem;
    }
  </style>
...

Then pair each class with a state:

...
<amp-state id="magicBox">
  <script type="application/json">
    {
      "visibleBox": {
        "className": "visible"
      },
      "invisibleBox": {
        "className": "invisible"
      },
      "moveLeft": {
        "className": "left"
      },
      "moveRight": {
        "className": "right"
      }
    }
  </script>
</amp-state>
...

And link the element with the classes:

...
  <div class=" " [class]="magicBox[animateBox].className"> </div>
...

The states change from a linked AMP action or event. The following example changes the state from user interaction:

...
<button on="tap:AMP.setState({animateBox: 'invisibleBox'})">
    Disappear
</button>
<button on="tap:AMP.setState({animateBox: 'visibleBox'})">
    Reappear
</button>
<button on="tap:AMP.setState({animateBox: 'moveLeft'})">
    Move Left
</button>
<button on="tap:AMP.setState({animateBox: 'moveRight'})">
  Move Right
</button>
...

Using amp-bind in this way set the class explicitly to the defined class. You will not have to tell it to remove other classes.