Zach’s ugly mug (his face) Zach Leatherman

Fire SVG animations (SMIL) when the SVG is visible

August 04, 2021

Had a use case come in where the design had an SVG animation that ran one interation and only one. It looked like this (some content has been removed):

If you’re on a device with a small vertical viewport size, you may not have noticed the above animation. Unfortunately the animations start (and complete!) whether or not the SVG is visible!

I wanted to wire the animations up to IntersectionObserver to make sure they only started animating when visible. Here’s how I did it:

Modify the SVG

I found all of the instances of <animate> or <animateTransform> and wired up the begin attributes to properly cascade the order of the animation internally. I want all of them to start when my bezier curve starts animating, so I added an id to that animation (there was an issue with dashes in that id, so beware using dashes):

+  id="mysvgline"

Learn how to animate a line/curve: css’s AvatarCSS Tricks: How SVG Line Animation Works

Take note of the begin attribute above, that will be important later.

Now I want to find the other animations in my SVG that I want to start at the same time and change their begin attribute to use the id from above with a .begin suffix. This starts this animation when the referenced animation starts. It looks like this:

+  begin="mysvgline.begin"
   path="M35.5 20C216.281 20 352.411 182 512.5 182"

Alternatively, you can use .end to start this animation when the referenced animation ends.

(Side note: repeatCount="1" and fill="freeze" are best buddies. fill="freeze" means that your animation won’t rewind to the first frame at the end)

Next go back to the original animation and change the begin attribute to indefinite (Read more at MDN: begin - SVG). This tells the SVG not to start it until I use JavaScript to trigger it using .beginElement().

+  begin="indefinite"

(Another side note: just thinking aloud here as I write this—I wonder if I can use <noscript> inside of SVG as a no-JS fallback)

Add the IntersectionObserver

It might look like this:

if ('IntersectionObserver' in window)  {
  // Recommended: make this selector more specific with a `data-animate-on-visible`
  let elements = document.querySelectorAll("svg");

  let observer = new IntersectionObserver(entries => {
    // quit early if users wants Reduced Motion
    let mediaQuery = window.matchMedia('(prefers-reduced-motion: no-preference)');
    if(!mediaQuery.matches) {

    for(let entry of entries) {
      if(!entry.isIntersecting) {

      // Look for <animate> or <animateTransform> that need JS to start
      let beginElements =`:scope [begin="indefinite"]`);
      for(let beginEl of beginElements) {

        // Unobserve so we don’t re-animate the dead
    threshold: .5 // 50% of element must be visible

  for(let elem of elements) {

Try it yourself

< Newer
Vector? Raster? Why Not Both!
Older >
If I work really hard on my Open Graph Images, People will share my Blog Posts

Zach Leatherman IndieWeb Avatar for a builder for the web and the creator/maintainer of IndieWeb Avatar for https://www.11ty.devEleventy (11ty), an award-winning open source site generator. At one point he became entirely too fixated on web fonts. He has given 81 talks in nine different countries at events like Beyond Tellerrand, Smashing Conference, Jamstack Conf, CSSConf, and The White House. Formerly part of CloudCannon, Netlify, Filament Group, NEJS CONF, and NebraskaJS. Learn more about Zach »


🔴 André Jaenisch 🔴David EastJohn Kemp-CruzNicolas HoizeytziCyberCode Twins👾🪐👾Thomas AllmerIndieWeb Avatar for https://mailsgun.ruFeedpushr feeds.IndieWeb Avatar for https://www.pixellyft.comVisionSnapIndieWeb Avatar for https://hnikoloski.comPawanjustJANICE1968IndieWeb Avatar for https://codinghindi.inSuzanne Pamela WebbIndieWeb Avatar for https://wpnewshub.comCOMMENTSENSEIndieWeb Avatar for https://www.67nj.orgNucléoloBryan D. Wilhite


Michael ZumsteinJacob LeechAlexei AccioPaul MeleroJim WolfRavin Sharmaハチ huangMukmin Abd SamadJosé Manuel DíazGeorg AndraeuptonkingMathieu Huotkeith kursonJohan TerpstraNikita Voloboevкг|sтоf вегиаегтEric WallaceMatt BiilmannTorsten KnabeRobTabsLeoaeeSam TancharoensuksavaiChen ZhixiangMatthias OttPaula DietermanSimon WillisonJason LengstorftziNicolas HoizeyAnna YeamanSia KaramalegosHyeseong KimAnikJeremy WynnDavid EastMichael SchultzThomas AllmerDennis FrankCyberCode Twins👾🪐👾Nic ColgrovePelle WessmanBen Delarre大島遼|Ryo Oshimakeith h.garlic salt enjoyerSimon St.LaurentAɴᴅʀᴇᴡ Sᴏʟᴀɴᴛᴏ🔴 André Jaenisch 🔴rong-senJohn Kemp-CruzAndy DaviesChris PortschellerAnthony Sapp
  1. Chris Portscheller


    SMIL is a thing again?

  2. Chris Portscheller


    I guess with IE becoming chromium it's relevant again.. which is awesome <3 I remember asking this on stackoverflow years ago...…

  3. Zach Leatherman


    oh yeah it never left—look at this beautiful green

  4. Chris Portscheller


    I really loved SMIL back in the day, but dropped it for a few projects because of this limitation in some enterprise projects. Definitely opens the doors for some new considerations.

  5. Zach Leatherman


    Honestly I only recently experimented with it but it’s working great!

  6. Guus Hoeve


    Forgive me for asking the obvious maybe, but what's the support for SMIL as I read it in Drasner's book SVG animations, but don't see anyone using it or in any animation tooling as export. I love SVG, no doubt, but I do want it to last.

  7. Zach Leatherman


    strange that the Edge switch to Chromium may have saved it? Note the warning on…

  8. Tabs


    I didn't bother with it much expecting it to disappear, but I guess now is a good time to dive right in?

  9. Guus Hoeve


    In my perception they are the same to SVG's animation attributes. If it's just a difference in name, it's already native I guess? Something we just don't think about anymore? Because otherwise, what is driving SVG animations natively? 😅

  10. Guus Hoeve


    One thing off the topic of SMIL, is that in SVG it's really sad you can't influence the scale of a displacementmap in the arguments like: <feDisplacementMap> ... scale="var(--scale)" ... so you can animate it in CSS. They need hardcoded values. That would … Truncated

  11. Sia Karamalegos


    "save it in my blog brain" is one of the top reasons I write blog posts 😆 I'm always referencing my own posts to remember how to do something

  12. Jon Kuperman


    I love your open graph images so much

  13. Zach Leatherman


    why thank you Jhon I wrote a blog post recently on this very topic

  14. Jon Kuperman


    I READ IT!!

  15. Zach Leatherman


    well that was your first mistake

Shamelessly plug your related post

These are webmentions via the IndieWeb and

Sharing on social media?

This is what will show up when you share this post on Social Media:

How did you do this? I automated my Open Graph images. (Peer behind the curtain at the test page)