15. Making images lazy-load with DevTools

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Lesson Details

  • Lesson: 15
  • Description: Using Local Overrides to add native lazy-loading to images

The goal of this lesson

To improve the performance of Dev Tips using native image lazy loading.

What we'll cover

  • Introduction
  • Lazy Loading Images
  • Local Overrides
  • Using a Web Proxy
  • Conclusion


Previously, the Dev Tips landing page loaded many thumbnail images on page load. These images were not lazily loaded.

Many Dev Tips Requests shown in the Network Panel

Lazy Loading Images

A search for mdn lazy loading eventually takes us here which explains the lazy loading syntax:

<img src="image.jpg" loading="lazy" alt="..." />

Local Overrides

You can configure Local Overrides in DevTools to support this workflow:

  1. Given a production-grade website, enable Local Overrides.
  2. Override the HTML so that the <img> tags are updated with the loading="lazy" attribute and value.
  3. Record a Network Profile of the overridden page to confirm only images within, or very close to the viewport, are loaded.
Local Overrides Overview

To use Local Overrides:

  1. Navigate to the Overrides pane in the Sources Panel.
  2. Select Enable Local Overrides.
  3. Select a folder on your filesystem where DevTools can save the changes you're making.
  4. Locate the resource you wish to edit in the Page pane (still within the Sources Panel).
  5. Edit the resource and save with Cmd + S / Ctrl + S.
  6. Reload your webpage, and observe your local resource is being used instead of the remote resource.

Using a Web Proxy

Various web proxy solutions can be installed on your machines to intercept requests, and serve a completely different resource - one which lives on your desktop for example. This means you can achieve the same thing as DevTools Local Overrides, but a proxy version will be compatible across all browsers, and also supports modifying requests and responses.

Charles Proxy

The Screenshot above comes from a tool known as Charles Proxy. HTTP & HTTPS interception is possible, and you can also point your mobile phone to the proxy installed on your machine, meaning overrides via a Proxy can be done on your phone too.


Local Overrides in DevTools offers a quick and easy way to map remote resources onto local resources, so you can prototype website variations, and performance profile them to verify they work.