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When JavaScript takes a long time to execute it can slow down website performance.

Duplicate JavaScript modules can slow down a website by requiring unnecessary downloads.

Inefficient animated content formats such as animated GIFs should be replaced by video.

This measures how long it takes a browser to render the first piece of content when someone visits a website.

Lazily loading above the fold images can negatively affect performance.

Avoid serving legacy JavaScript to modern browsers, only serving it to older browsers that require it.

When the main thread is busy the website may not respond quickly to user interactions.

This measures the time from when a user first interacts with a website to the time when the browser is actually able to respond to that interaction.

External scripts dynamically injected via document.write() can delay page load by tens of seconds.

Lazy-loading offscreen and hidden images after all critical resources have finished loading can significantly increase website performance.

The image used by the largest contentful paint element should be preloaded to maximize performance.

Avoid multiple page redirects as they introduce additional delays before the page can be loaded.

This indicates how quickly the contents of a page are visibly populated.

Excessively large page sizes can lead to long load times for visitors.

Minifying CSS files can reduce payload sizes.

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

Reduce unused rules from stylesheets to decrease bytes consumed by network activity.

Optimized images load faster and consume less network resources.

Responsive images that are appropriately-sized save network resources and improve load time.

Text-based resources can be served with compression (gzip, deflate or brotli) to minimize network load.

Incorrect character encoding can result in garbled text and is properly declared in the charset attribute or the Content-Type HTTP response header.

Specifying a doctype prevents the browser from switching to quirks mode, which can cause a website to render in unexpected ways.

Ensure text is user-visible and avoid flashes of invisible text while webfonts are loading.

Users are mistrustful or confused by websites that request their location without context.

Images displayed with an incorrect aspect ratio look distorted.

Images should be responsive and proportional to the display size and the pixel ratio to maximize image clarity.

Unload event listeners do not fire reliably and listening for them can intefere with website functionality and browser optimizations.

Users are mistrustful of or confused by sites that request to send notifications without context.

Correctly setting the viewport optimizes websites for mobile screen sizes and also prevents delays to user input.

Deprecated APIs are scheduled to be removed from browsers and can introduce errors and vulnerabilities.

Console errors indicate unresolved problems that can lead to security issues.

All websites should be protected with HTTPS including ones that don't handle sensitive data.

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

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

Lists must only contain list elements so that screen readers can correctly announce lists.

List items must be contained within a parent list for screen readers to properly announce lists.

A tabindex value greater than 0 implies an explicit navigation ordering which often creates frustrating experiences for users who rely on assistive technologies.

Specifying a valid lang attributes ensure that text is pronounced correctly by a screen reader.

Valid canonical links tell search engines which version of a page to crawl and display to users in search results.

Search engine users rely on the document title to determine if a page is relevant to their search.

Many sites provide different versions of a page based on a user's language or region and hreflang links tell search engines the URLs for all the versions of a page so that they can display the correct version.

Pages with unsuccessful HTTP status codes may not be indexed by search engines properly.

Search engines are unable to include websites in search results if they don't have permission to crawl them.

Descriptive link text helps search engines understand your content.

Meta descriptions may be included in search results to help convey website content.

Search engines can't index plugin content and many devices restrict plugins or don't support them.

A valid robots.txt is required for search engines to understand how to crawl or index a website.