Zach’s ugly mug (his face) Zach Leatherman

Ruthlessly Eliminating Layout Shift on

November 25, 2020

Updated on 17 November 2023 to simplify the CSS to use :defined

On the Netlify web site, we have a little banner that appears at the top to drive traffic to new and exciting things happening in Netlify-land.

Screenshot of the banner on

That banner has exactly two features:

  1. An advanced HTML feature known only to a select few Old Guard developers: the hyperlink.
  2. A close button (which saves the preference for future page loads)

There are a few key performance milestones in the lifecycle of this component, and this is how worked previously:

  1. The page’s initial render. The banner is ⚠️⚠️⚠️ hidden by default. Without JavaScript or before JavaScript loads, the banner is hidden.
  2. After the JavaScript loads, we check localStorage to see if the user has closed the banner previously. We hash this preference to the banner URL so that if the banner changes, it will render even if the user has opted out previously. If applicable, render the banner.
  3. Lastly we bind JavaScript events to the close button. Events are not necessary for the hyperlink, because its behavior is delivered exlusively in HTML (very much wow).

Steps 2 and 3 were bundled and executed together in the same component code file. And in some earlier iterations of the site, up the amount of time that elapsed between Step 1 and 2 could be up to ~600 ms.

Filmstrip showing hidden banner for ~600ms on old design

On our new site (faster, mind you) redesign we inlined the JavaScript for Steps 2 and 3 into the end of the <body> and the delay was still very present:

Filmstrip showing hidden banner for ~600ms on the new design

The fix

What we needed to do was swap the behavior. The common use case was to visit the site without the opt-in preference to hide the banner. We must make the banner visible by default and make the JavaScript path to hide it the exception to the rule.

This changes our previously mentioned Step 1, the page’s initial render, show the banner. Without JavaScript or before JavaScript loads, the banner should be visible.

We also split the JavaScript code for the component into two separate pieces: one piece to check whether or not the user has the preference to hide and a separate Web Component to bind the events.

Update: I’ve packaged up the code below and put it on GitHub for re-use.


We use opacity to toggle the close button so that it doesn’t reflow the component when it’s enabled via JavaScript.

.banner--hide announcement-banner,
announcement-banner[hidden] {
  display: none;
/* The close button will not be visible until the component is registered,
 * preventing ghost clicks on the button before the event listener is added.
announcement-banner:not(:defined) [data-banner-close] {
  opacity: 0;
  pointer-events: none;
  <a href="">Read about our Sustainability</a>
  <button type="button" data-banner-close>Close</button>


banner-helper.js, put into the <head>:

// the current banner CTA URL, we inject this from a JSON data file
let ctaUrl = "";
let savedCtaUrl = localStorage.getItem("banner--cta-url");

if(savedCtaUrl === ctaUrl) {

banner.js, defer this until later (how much later is up to you):

class Banner extends HTMLElement {
  connectedCallback() {
    let button = this.querySelector("[data-banner-close]");
    if(button) {
      button.addEventListener("click", () => {

  savePreference() {
    let cta = this.querySelector("a[href]");
    if(cta) {
      let ctaUrl = cta.getAttribute("href");
      localStorage.setItem("banner--cta-url", ctaUrl);

  close() {
    this.setAttribute("hidden", true);

window.customElements.define("announcement-banner", Banner);

Astute readers will notice that the above is a web component but let’s just keep that between us.

The Results

Note that the first render contains the banner! This is the same render behavior whether or not JavaScript is in play.

Filmstrip showing banner visible on first render

In this waterfall comparison, you might note that we reduced layout shift metrics to zero.

Graph of Layout Shifts: previous has .35 and new has 0

And because we inlined the script for repeat views into the <head>, when you hide the banner and navigate to a new page, the banner will be hidden before first render too.

Not too bad for a few small changes!

Next target will be to improve the web font render.

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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 »


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  1. Phil Wolstenholme


    Thanks for writing this up, I've just changed my approach on a project I'm working on now as a result of seeing the way you're using JS (but in the `head`) to control visibility 👍

  2. Zach Leatherman


    Awesome! 👍🏻

  3. Jouni Kantola


    Would there have been any performance upside or downside by running the class toggle script in the web component instead to not have blocking code in head?

  4. Zach Leatherman


    Yeah, that’s too late. The second filmstrip in the post showed similar behavior

  5. Jouni Kantola


    All right! Thanks for the writeup. By that, I gather head is the way to go for light/dark mode toggles as well.

  6. Lukas Grebe


    I don’t necessarily understand banners. I’ve navigated to a specific content for some reason. Why distract and point my attention at some other very specific content, instead of maybe pointing out this content AFTER the content I originally came for?

  7. Lukas Grebe


    a fun anecdote: I heard you like banners so we put a banner on the landing page of the banner so you can click the banner to see another banner!

  8. Zach Leatherman


    Yeah I’d wager that too 👍🏻

  9. Lukas Grebe


    Look: German has a name for itörer_(W… Still. I wonder if the very first impression shouldn’t be a Störer but something else.

  10. Søren Birkemeyer 🦊


    Because marketing.

  11. Lukas Grebe


    Absolutely and this is only a tangent to the technical discussion. (Which w/o Marketing wouldn’t be needed) Im saying a general banner for everyone on every page (even the landing page of the banner) might not be the best way to go.

  12. Lukas Grebe


    FTA:“The common use case was to visit the site without the opt-in preference to hide the banner. We must make the banner visible by default“ - I’m saying this premise might not be complete

  13. Lukas Grebe


    I ❤️ netlify and marketing IS marketing 🤷‍♂️ So let’s just leave it at „yay for less content shifting and a faster web“ 🥰

  14. Zach Leatherman


    ❤️ I think it’s a fair point! Just for the technical discussion about hiding the banner on the target page, I’m also thinking about the benefit of navigation stability on navigation between pages.

  15. Zach Leatherman


    The other complaint I currently have about our banner is that the small viewport nav close button isn’t in the same place as the open hamburger icon when the banner is open 😱

  16. Šime Vidas


    This sounds like a use-case for progressive rendering.

  17. Zach Leatherman


    Is this separate from the normal progressive rendering behavior of HTML

  18. Šime Vidas


    Basically, splitting the page into parts and loading, executing, and rendering each part separately in sequential order. <!-- header --> <link rel="stylesheet" href="header.css"> <script src="header.js"> <header> … </header> … Truncated

  19. Stoyan Stefanov


    hm, and your feed broke Looking... :)

  20. Zach Leatherman


    😅 sorry!

  21. Stoyan Stefanov


    haha, I think I got it! My 500 char substring cut off the ⚠️ emoji so json_encode couldn't handle it…

  22. cthos


    We should be sharing the good word of webcomponents. Allllll the good words.... O_O

  23. Zach Leatherman


    Can I curse too or are the bad words off limits

  24. cthos


    I do so regularly, so yes.

  25. Adam Duncan


    @zachleat did a write-up on how they tackled it at Netlify a little while back:…

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