Lazy loading

Lazy loading is a strategy to identify resources as non-blocking (non-critical) and load these only when needed. It's a way to shorten the length of the critical rendering path, which translates into reduced page load times.

Lazy loading can occur on different moments in the application, but it typically happens on some user interactions such as scrolling and navigation. 

Overview

As the web has evolved, we have come to see huge increases in the number and size of assets sent to users.
Between 2011 and 2019, the median resource weight increased from ~100KB to ~400KB for desktop and ~50KB to ~350KB for mobile. While Image size has increased from ~250KB to ~900KB on desktop and ~100KB to ~850KB on mobile.

One of the methods we can use to tackle this problem is to shorten the Critical Rendering Path length by lazy loading resources that are not critical for the first render to happen.
A practical example would be when, you land on the home page of an e-commerce site which has a link to a cart page/section and all its resources (JS, CSS, images...) are downloaded only when the user navigates to that cart page.

Strategies

Lazy loading can be applied to multiple resources and through multiple strategies.   

    General

Code splitting
JavaScript, CSS and HTML can be split into smaller chunks. This enables sending the minimal code required to provide value upfront, improving page-load times. The rest can be loaded on demand.

  •     Entry point splitting: separates code by entry point(s) in the app
  •     Dynamic splitting: separates code where dynamic import() statements are used

    JavaScript

Script type module
Any script tag with type="module" is treated like a JavaScript module and is deferred by default.

    CSS

By default, CSS is treated as a render blocking resource, so the browser won't render any processed content until the CSSOM is constructed. CSS must be thin, delivered as quickly as possible, and the usage media types and queries are advised to unblock rendering.

<link href="style.css"    rel="stylesheet" media="all">
<link href="portrait.css" rel="stylesheet" media="orientation:portrait">
<link href="print.css"    rel="stylesheet" media="print">

It is possible to perform some CSS optimizations  to achieve that.

Fonts

By default, font requests are delayed until the render tree is constructed, which can result in delayed text rendering.

It is possible to override the default behaviour and preload web font resources using <link rel="preload">, the CSS font-display property, and the Font Loading API.

See also: Element Link

Images and iframes

Very often, webpages contain many images that contribute to data-usage and how fast a page can load. Most of those images are off-screen (non-critical), requiring user interaction (an example being scroll) in order to view them.

Loading attribute
The loading attribute on an <img> element (or the loading attribute on an <iframe>) can be used to instruct the browser to defer loading of images/iframes that are off-screen until the user scrolls near them.

<img src="image.jpg" loading="lazy" alt="..." />
<iframe src="video-player.html" loading="lazy"></iframe>

The load event fires when the eagerly-loaded content has all been loaded; at that time, it's entirely possible (or even likely) that there may be lazily-loaded images that are within the visual viewport that haven't yet loaded.

You can determine if a given image has finished loading by examining the value of its Boolean complete property.

Polyfill
Include this polyfill to provide support for older and currently incompatible browsers:
loading-attribute-polyfill

Intersection Observer API
Intersection Observers allow the user to know when an observed element enters or exits the browser’s viewport.

Event handlers
When browser compatibility is crucial, there are a few options:

Specifications

Specification Status Comment
HTML Living Standard Living Standard

See also