First Contentful Paint

First Contentful Paint marks the time at which the first text or image is painted.

Speed Index

Speed Index shows how quickly the contents of a page are visibly populated.

Largest Contentful Paint

Largest Contentful Paint marks the time at which the largest text or image is painted.

Time to Interactive

Time to interactive is the amount of time it takes for the page to become fully interactive.

Total Blocking Time

Sum of all time periods between FCP and Time to Interactive, when task length exceeded 50ms, expressed in milliseconds.

Cumulative Layout Shift

Cumulative Layout Shift measures the movement of visible elements within the viewport.

Max Potential First Input Delay

The maximum potential First Input Delay that your users could experience is the duration of the longest task.

First Meaningful Paint

First Meaningful Paint measures when the primary content of a page is visible.

Eliminate render-blocking resources

Resources are blocking the first paint of your page. Consider delivering critical JS/CSS inline and deferring all non-critical JS/styles.

Properly size images

Serve images that are appropriately-sized to save cellular data and improve load time.

Defer offscreen images

Consider lazy-loading offscreen and hidden images after all critical resources have finished loading to lower time to interactive.

Minify CSS

Minifying CSS files can reduce network payload sizes.

Minify JavaScript

Minifying JavaScript files can reduce payload sizes and script parse time.

Reduce unused CSS

Reduce unused rules from stylesheets and defer CSS not used for above-the-fold content to decrease bytes consumed by network activity.

Reduce unused JavaScript

Reduce unused JavaScript and defer loading scripts until they are required to decrease bytes consumed by network activity.

Efficiently encode images

Optimized images load faster and consume less cellular data.

Serve images in next-gen formats

Image formats like WebP and AVIF often provide better compression than PNG or JPEG, which means faster downloads and less data consumption.

Enable text compression

Text-based resources should be served with compression (gzip, deflate or brotli) to minimize total network bytes.

Preconnect to required origins

Consider adding `preconnect` or `dns-prefetch` resource hints to establish early connections to important third-party origins.

Initial server response time was short

Keep the server response time for the main document short because all other requests depend on it.

Avoid multiple page redirects

Redirects introduce additional delays before the page can be loaded.

Preload key requests

Consider using `<link rel=preload>` to prioritize fetching resources that are currently requested later in page load.

Use HTTP/2

HTTP/2 offers many benefits over HTTP/1.1, including binary headers and multiplexing.

Use video formats for animated content

Large GIFs are inefficient for delivering animated content. Consider using MPEG4/WebM videos for animations and PNG/WebP for static images instead of GIF to save network bytes.

Remove duplicate modules in JavaScript bundles

Remove large, duplicate JavaScript modules from bundles to reduce unnecessary bytes consumed by network activity.

Avoid serving legacy JavaScript to modern browsers

Polyfills and transforms enable legacy browsers to use new JavaScript features. However, many aren't necessary for modern browsers. For your bundled JavaScript, adopt a modern script deployment strategy using module/nomodule feature detection to reduce the amount of code shipped to modern browsers, while retaining support for legacy browsers. [Learn More](https://philipwalton.com/articles/deploying-es2015-code-in-production-today/)

Preload Largest Contentful Paint image

Preload the image used by the LCP element in order to improve your LCP time.

Avoids enormous network payloads

Large network payloads cost users real money and are highly correlated with long load times.

Uses efficient cache policy on static assets

A long cache lifetime can speed up repeat visits to your page.

Avoids an excessive DOM size

A large DOM will increase memory usage, cause longer [style calculations](https://developers.google.com/web/fundamentals/performance/rendering/reduce-the-scope-and-complexity-of-style-calculations), and produce costly [layout reflows](https://developers.google.com/speed/articles/reflow).

Avoid chaining critical requests

The Critical Request Chains below show you what resources are loaded with a high priority. Consider reducing the length of chains, reducing the download size of resources, or deferring the download of unnecessary resources to improve page load.

User Timing marks and measures

Consider instrumenting your app with the User Timing API to measure your app's real-world performance during key user experiences.

JavaScript execution time

Consider reducing the time spent parsing, compiling, and executing JS. You may find delivering smaller JS payloads helps with this.

Minimizes main-thread work

Consider reducing the time spent parsing, compiling and executing JS. You may find delivering smaller JS payloads helps with this.

All text remains visible during webfont loads

Leverage the font-display CSS feature to ensure text is user-visible while webfonts are loading.

Performance budget

Keep the quantity and size of network requests under the targets set by the provided performance budget.

Timing budget

Set a timing budget to help you keep an eye on the performance of your site. Performant sites load fast and respond to user input events quickly.

Keep request counts low and transfer sizes small

To set budgets for the quantity and size of page resources, add a budget.json file.

Minimize third-party usage

Third-party code can significantly impact load performance. Limit the number of redundant third-party providers and try to load third-party code after your page has primarily finished loading.

Lazy load third-party resources with facades

Some third-party embeds can be lazy loaded. Consider replacing them with a facade until they are required.

Largest Contentful Paint element

This is the largest contentful element painted within the viewport. [Learn More](https://web.dev/lighthouse-largest-contentful-paint/)

Avoid large layout shifts

These DOM elements contribute most to the CLS of the page.

Uses passive listeners to improve scrolling performance

Consider marking your touch and wheel event listeners as `passive` to improve your page's scroll performance.

Avoids `document.write()`

For users on slow connections, external scripts dynamically injected via `document.write()` can delay page load by tens of seconds.

Avoid long main-thread tasks

Lists the longest tasks on the main thread, useful for identifying worst contributors to input delay.

Avoid non-composited animations

Animations which are not composited can be janky and increase CLS.

Image elements have explicit `width` and `height`

Set an explicit width and height on image elements to reduce layout shifts and improve CLS.

Network Requests

Lists the network requests that were made during page load.

Network Round Trip Times

Network round trip times (RTT) have a large impact on performance. If the RTT to an origin is high, it's an indication that servers closer to the user could improve performance.

Server Backend Latencies

Server latencies can impact web performance. If the server latency of an origin is high, it's an indication the server is overloaded or has poor backend performance.

Tasks

Lists the toplevel main thread tasks that executed during page load.

Diagnostics

Collection of useful page vitals.

Metrics

Collects all available metrics.

Screenshot Thumbnails

This is what the load of your site looked like.

Final Screenshot

The last screenshot captured of the pageload.

Script Treemap Data

Used for treemap app

`[accesskey]` values are unique

Access keys let users quickly focus a part of the page. For proper navigation, each access key must be unique.

`[aria-*]` attributes match their roles

Each ARIA `role` supports a specific subset of `aria-*` attributes. Mismatching these invalidates the `aria-*` attributes.

`button`, `link`, and `menuitem` elements have accessible names

When an element doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

`[aria-hidden="true"]` is not present on the document `<body>`

Assistive technologies, like screen readers, work inconsistently when `aria-hidden="true"` is set on the document `<body>`.

`[aria-hidden="true"]` elements do not contain focusable descendents

Focusable descendents within an `[aria-hidden="true"]` element prevent those interactive elements from being available to users of assistive technologies like screen readers.

ARIA input fields have accessible names

When an input field doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

ARIA `meter` elements have accessible names

When an element doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

ARIA `progressbar` elements have accessible names

When a `progressbar` element doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

`[role]`s have all required `[aria-*]` attributes

Some ARIA roles have required attributes that describe the state of the element to screen readers.

Elements with an ARIA `[role]` that require children to contain a specific `[role]` have all required children.

Some ARIA parent roles must contain specific child roles to perform their intended accessibility functions.

`[role]`s are contained by their required parent element

Some ARIA child roles must be contained by specific parent roles to properly perform their intended accessibility functions.

`[role]` values are valid

ARIA roles must have valid values in order to perform their intended accessibility functions.

ARIA toggle fields have accessible names

When a toggle field doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

ARIA `tooltip` elements have accessible names

When an element doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

ARIA `treeitem` elements have accessible names

When an element doesn't have an accessible name, screen readers announce it with a generic name, making it unusable for users who rely on screen readers.

`[aria-*]` attributes have valid values

Assistive technologies, like screen readers, can't interpret ARIA attributes with invalid values.

`[aria-*]` attributes are valid and not misspelled

Assistive technologies, like screen readers, can't interpret ARIA attributes with invalid names.

Buttons have an accessible name

When a button doesn't have an accessible name, screen readers announce it as "button", making it unusable for users who rely on screen readers.

The page contains a heading, skip link, or landmark region

Adding ways to bypass repetitive content lets keyboard users navigate the page more efficiently.

Background and foreground colors have a sufficient contrast ratio

Low-contrast text is difficult or impossible for many users to read.

`<dl>`'s contain only properly-ordered `<dt>` and `<dd>` groups, `<script>`, `<template>` or `<div>` elements.

When definition lists are not properly marked up, screen readers may produce confusing or inaccurate output.

Definition list items are wrapped in `<dl>` elements

Definition list items (`<dt>` and `<dd>`) must be wrapped in a parent `<dl>` element to ensure that screen readers can properly announce them.

Document has a `<title>` element

The title gives screen reader users an overview of the page, and search engine users rely on it heavily to determine if a page is relevant to their search.

`[id]` attributes on active, focusable elements are unique

All focusable elements must have a unique `id` to ensure that they're visible to assistive technologies.

ARIA IDs are unique

The value of an ARIA ID must be unique to prevent other instances from being overlooked by assistive technologies.

No form fields have multiple labels

Form fields with multiple labels can be confusingly announced by assistive technologies like screen readers which use either the first, the last, or all of the labels.

`<frame>` or `<iframe>` elements have a title

Screen reader users rely on frame titles to describe the contents of frames.

Heading elements appear in a sequentially-descending order

Properly ordered headings that do not skip levels convey the semantic structure of the page, making it easier to navigate and understand when using assistive technologies.

`<html>` element has a `[lang]` attribute

If a page doesn't specify a lang attribute, a screen reader assumes that the page is in the default language that the user chose when setting up the screen reader. If the page isn't actually in the default language, then the screen reader might not announce the page's text correctly.

`<html>` element has a valid value for its `[lang]` attribute

Specifying a valid [BCP 47 language](https://www.w3.org/International/questions/qa-choosing-language-tags#question) helps screen readers announce text properly.

Image elements have `[alt]` attributes

Informative elements should aim for short, descriptive alternate text. Decorative elements can be ignored with an empty alt attribute.

`<input type="image">` elements have `[alt]` text

When an image is being used as an `<input>` button, providing alternative text can help screen reader users understand the purpose of the button.

Form elements have associated labels

Labels ensure that form controls are announced properly by assistive technologies, like screen readers.

Links have a discernible name

Link text (and alternate text for images, when used as links) that is discernible, unique, and focusable improves the navigation experience for screen reader users.

Lists contain only `<li>` elements and script supporting elements (`<script>` and `<template>`).

Screen readers have a specific way of announcing lists. Ensuring proper list structure aids screen reader output.

List items (`<li>`) are contained within `<ul>` or `<ol>` parent elements

Screen readers require list items (`<li>`) to be contained within a parent `<ul>` or `<ol>` to be announced properly.

The document does not use `<meta http-equiv="refresh">`

Users do not expect a page to refresh automatically, and doing so will move focus back to the top of the page. This may create a frustrating or confusing experience.

`[user-scalable="no"]` is not used in the `<meta name="viewport">` element and the `[maximum-scale]` attribute is not less than 5.

Disabling zooming is problematic for users with low vision who rely on screen magnification to properly see the contents of a web page.

`<object>` elements have `[alt]` text

Screen readers cannot translate non-text content. Adding alt text to `<object>` elements helps screen readers convey meaning to users.

No element has a `[tabindex]` value greater than 0

A value greater than 0 implies an explicit navigation ordering. Although technically valid, this often creates frustrating experiences for users who rely on assistive technologies.

Cells in a `<table>` element that use the `[headers]` attribute refer to table cells within the same table.

Screen readers have features to make navigating tables easier. Ensuring `<td>` cells using the `[headers]` attribute only refer to other cells in the same table may improve the experience for screen reader users.

`<th>` elements and elements with `[role="columnheader"/"rowheader"]` have data cells they describe.

Screen readers have features to make navigating tables easier. Ensuring table headers always refer to some set of cells may improve the experience for screen reader users.

`[lang]` attributes have a valid value

Specifying a valid [BCP 47 language](https://www.w3.org/International/questions/qa-choosing-language-tags#question) on elements helps ensure that text is pronounced correctly by a screen reader.

`<video>` elements contain a `<track>` element with `[kind="captions"]`

When a video provides a caption it is easier for deaf and hearing impaired users to access its information.

The page has a logical tab order

Tabbing through the page follows the visual layout. Users cannot focus elements that are offscreen.

Interactive controls are keyboard focusable

Custom interactive controls are keyboard focusable and display a focus indicator.

Interactive elements indicate their purpose and state

Interactive elements, such as links and buttons, should indicate their state and be distinguishable from non-interactive elements.

The user's focus is directed to new content added to the page

If new content, such as a dialog, is added to the page, the user's focus is directed to it.

User focus is not accidentally trapped in a region

A user can tab into and out of any control or region without accidentally trapping their focus.

Custom controls have associated labels

Custom interactive controls have associated labels, provided by aria-label or aria-labelledby.

Custom controls have ARIA roles

Custom interactive controls have appropriate ARIA roles.

Visual order on the page follows DOM order

DOM order matches the visual order, improving navigation for assistive technology.

Offscreen content is hidden from assistive technology

Offscreen content is hidden with display: none or aria-hidden=true.

HTML5 landmark elements are used to improve navigation

Landmark elements (<main>, <nav>, etc.) are used to improve the keyboard navigation of the page for assistive technology.

Uses HTTPS

All sites should be protected with HTTPS, even ones that don't handle sensitive data. This includes avoiding [mixed content](https://developers.google.com/web/fundamentals/security/prevent-mixed-content/what-is-mixed-content), where some resources are loaded over HTTP despite the initial request being served over HTTPS. HTTPS prevents intruders from tampering with or passively listening in on the communications between your app and your users, and is a prerequisite for HTTP/2 and many new web platform APIs.

Links to cross-origin destinations are safe

Add `rel="noopener"` or `rel="noreferrer"` to any external links to improve performance and prevent security vulnerabilities.

Avoids requesting the geolocation permission on page load

Users are mistrustful of or confused by sites that request their location without context. Consider tying the request to a user action instead.

Avoids requesting the notification permission on page load

Users are mistrustful of or confused by sites that request to send notifications without context. Consider tying the request to user gestures instead.

Ensure CSP is effective against XSS attacks

A strong Content Security Policy (CSP) significantly reduces the risk of cross-site scripting (XSS) attacks.

Allows users to paste into password fields

Preventing password pasting undermines good security policy.

Displays images with correct aspect ratio

Image display dimensions should match natural aspect ratio.

Serves images with appropriate resolution

Image natural dimensions should be proportional to the display size and the pixel ratio to maximize image clarity.

Fonts with `font-display: optional` are preloaded

Preload `optional` fonts so first-time visitors may use them.

Page has the HTML doctype

Specifying a doctype prevents the browser from switching to quirks-mode.

Properly defines charset

A character encoding declaration is required. It can be done with a `<meta>` tag in the first 1024 bytes of the HTML or in the Content-Type HTTP response header.

Avoids `unload` event listeners

The `unload` event does not fire reliably and listening for it can prevent browser optimizations like the Back-Forward Cache. Consider using the `pagehide` or `visibilitychange` events instead.

Avoids Application Cache

Application Cache is deprecated.

Avoids deprecated APIs

Deprecated APIs will eventually be removed from the browser.

No browser errors logged to the console

Errors logged to the console indicate unresolved problems. They can come from network request failures and other browser concerns.

Page has valid source maps

Source maps translate minified code to the original source code. This helps developers debug in production. In addition, Lighthouse is able to provide further insights. Consider deploying source maps to take advantage of these benefits.

No issues in the `Issues` panel in Chrome Devtools

Issues logged to the `Issues` panel in Chrome Devtools indicate unresolved problems. They can come from network request failures, insufficient security controls, and other browser concerns. Open up the Issues panel in Chrome DevTools for more details on each issue.

Registers a service worker that controls page and `start_url`

The service worker is the technology that enables your app to use many Progressive Web App features, such as offline, add to homescreen, and push notifications.

Redirects HTTP traffic to HTTPS

If you've already set up HTTPS, make sure that you redirect all HTTP traffic to HTTPS in order to enable secure web features for all your users.

Sets a theme color for the address bar.

The browser address bar can be themed to match your site.

Content is sized correctly for the viewport

If the width of your app's content doesn't match the width of the viewport, your app might not be optimized for mobile screens.

Has a `<meta name="viewport">` tag with `width` or `initial-scale`

Add a `<meta name="viewport">` tag to optimize your app for mobile screens.

Provides a valid `apple-touch-icon`

For ideal appearance on iOS when users add a progressive web app to the home screen, define an `apple-touch-icon`. It must point to a non-transparent 192px (or 180px) square PNG. [Learn More](https://web.dev/apple-touch-icon/).

Site works cross-browser

To reach the most number of users, sites should work across every major browser.

Page transitions don't feel like they block on the network

Transitions should feel snappy as you tap around, even on a slow network. This experience is key to a user's perception of performance.

Each page has a URL

Ensure individual pages are deep linkable via URL and that URLs are unique for the purpose of shareability on social media.