General

New Core Web Vitals and How They Work

Published Feb 14, 2024

Updated Feb 16, 2024

6 min read

Core Web Vitals, introduced by Google in 2020, have been around for a while now. And after four years, a significant change is coming in March 2024. In this article, I will try to tell you everything you need to know to be prepared. But first, let's briefly talk about what Core Web Vitals are.

What are Core Web Vitals?

Web Vitals is a Google initiative that aims to provide developers with guidance on authoring the best user experience on the web as well as tools to assess and measure it. Core Web Vitals are a subset of Web Vitals and currently consist of three main metrics:

Largest Contentful Paint (LCP): This metric measures the time it takes for the largest content element to be rendered on the screen. It should reflect the loading aspect of the user experience.
First Input Delay (FID): This metric measures the time it takes for the browser to respond to the first user interaction. It should reflect the interactivity aspect of the user experience.
Cumulative Layout Shift (CLS): This metric measures the amount of unexpected layout shifts of visual elements on the page. It should reflect the visual stability aspect of the user experience.

Part of the Core Web Vitals are also the thresholds that define what is considered a good experience. These thresholds are currently set to:

Metric	Good	Needs Improvement	Bad
LCP	≤ 2.5 s	2.5 - 4 s	> 4 s
FID	≤ 100 ms	100 - 300 ms	> 300 ms
CLS	≤ 0.1	0.1 - 0.25	> 0.25

While the reward of providing a good user experience should be enough for you to strive for great results in these metrics, that is not the only reason you should care about them. Google is reflecting these metrics in its search ranking algorithm as it wants it to "reward content that offers good user experience".

What's Changing in March 2024?

The change affects how interactivity is measured. The First Input Delay (FID) has been deemed no longer sufficient to measure the interactivity aspect of the user experience. It will be replaced by the Interaction to Next Paint (INP) metric on March 12, 2024.

INP, unlike FID, measures the response of all interactions up to a URL change. It then picks the slowest response from all interactions. (If there are more than 50 interactions, it picks a percentile instead, most often the 98th) making it much stricter than FID.

Why the Change?

There are three main reasons why FID is being replaced by INP:

FID is not strict enough: Upwards of 90% of websites meet the good threshold for FID nowadays, making it a poor differentiator between good and bad user experiences.
FID is not representative of the full interaction: FID only measures the first part of the reaction delay, up to the point before the response bubbles to JavaScript, not accounting for the (often quite long) time it takes for the JavaScript to process the response and for the browser to paint the next frame.
FID only measures the first interaction: According to Google, 90% of interactions happen after the first load, making FID not representative of the full user experience.

INP aims to address these issues by measuring the full interaction and by being stricter in its thresholds (according to Google, ~70% of websites should pass the good INP threshold now) and thus capturing a more representative picture of the user experience.

How Does INP Work?

As I previously mentioned, INP measures all interactions on the page and picks the slowest one or a percentile, which solves one of the issues with FID not representing the whole experience of interacting with a website. This way of measuring is not dissimilar to how CLS measures the unexpected layout shifts on the page as it also captures the full scope of the stay on the page.

The interactions considered by INP are:

Mouse clicks
Touchscreen taps
Keyboard presses (including virtual onscreen keyboards)

If an interaction is composed of multiple events, such as a keystroke, consisting of the keydown, keypress, and keyup events; INP groups them and measures the latency as the maximum duration of all the included events.

Unlike FID, it captures not only the time taken by tasks blocked by the main thread (the input delay) but also the processing time (events and JavaScript processing) and the presentation delay (the time it takes for the browser to paint the next frame), making it a much better reflection of the full user experience, but also much harder to pass.

So while FID reflects the first impression, INP should capture the overall responsiveness of a page.

For more information about interactions with comprehensive diagrams of what actually happens in the browser, check out the web.dev article on INP.

Google defined the thresholds for INP as follows:

Metric	Good	Needs Improvement	Bad
INP	≤ 200 ms	200 - 500 ms	> 500 ms

How to Prepare for INP?

Let's start with some general steps you should take to prepare for the new Web Vital:

1. Measure

Start with measuring your INP score. The easiest way to measure it is using PageSpeed Insights, but if you need to measure it locally, you can use Lighthose user flows which already support INP. If your INP scores are good, you're fine and you can probably stop reading further.

2. Do not panic

While, according to Tim Kadlec, only 17% of React websites and 11.8% of Next.js websites in the "top 100k" pass the good INP threshold, so it's very likely that you will have to do some work to pass; framework authors have been working on improving their performance and things may be as hot as Tim paints them. According to Google’s stats, ~80-90% of desktop sites pass the good INP threshold depending on the framework as of June 2023:

Technology	Passing on Mobile [%]	Passing on Desktop [%]
Angular (v2.0.0+)	28.6	83.6
Next.js	28.5	87.3
Nuxt.js	32.0	91.2
Preact	48.6	92.8
Vue (v2.0.0+)	50.3	94.1
Lit	50.0	88.3

3. Optimize

In case your INP scores are not good, you should start optimizing your website. Identify the pages that perform the worst and start with them. Focus on:

Code splitting and tree shaking to make sure only the necessary JavaScript is included.
Deferring non-critical JavaScript events until the main thread is idle. Consider also offloading heavy tasks to a Web Worker.
Debouncing or throttling frequently fired events.
Breaking up long JavaScript tasks into smaller ones.
Using passive event listeners.
Reducing the presentational delay e.g. by using the will-change CSS property, and preferring transform and opacity over the display and position properties in animations.
Eliminating third-party scripts. Very often, third-party scripts, be it for analytics, cookie consent, or other purposes, have a significant impact on performance. Considering which of them are not absolutely necessary, and either removing them or replacing them with a more performant solution is highly recommended.
Upgrading your dependencies to make sure you are taking advantage of the latest performance-improving features.

If you want to read more about the general INP optimization tips, you can check out this Vercel article.

While a lot of these optimizations, such as code splitting and tree shaking, are already taken care of by modern frameworks, I believe that it’s worth going through this list to see what you can do to make your website faster and thus a better experience for your users. We will explore concrete performance tips for popular frameworks in the next article.

Conclusion

The change in Core Web Vitals is coming and it's going to be a significant one. The new metric, Interaction to Next Paint, is going to be much stricter than the current First Input Delay and it's going to be much harder to pass. However, the good news is that framework authors have already been working on making the frameworks more INP-friendly, and while it may take some effort, you should be able to improve your INP score.

INP, however, should not be just another Lighthouse threshold we have to satisfy. This change should encourage us to look beyond surface-level metrics and dive deeper into the nuances of user interaction. As we adapt to these changes, our focus should remain on the end goal: creating a good and inclusive user experience for our visitors.

This Dot is a consultancy dedicated to guiding companies through their modernization and digital transformation journeys. Specializing in replatforming, modernizing, and launching new initiatives, we stand out by taking true ownership of your engineering projects.

We love helping teams with projects that have missed their deadlines or helping keep your strategic digital initiatives on course. Check out our case studies and our clients that trust us with their engineering.

About the author(s)

Jan Kaiser
Software engineer passionate about everything web-related. Particularly loves Angular, SCSS, TypeScript, soccer and dachshunds
@honzikec @honzikec

The HTML Dialog Element: Enhancing Accessibility and Ease of Use

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This is helpful as it reduces the burden on the developer when making their applications accessible by ensuring that user expectations concerning interaction are met, and it can also potentially simplify the implementation of dialogs in general. Basic usage Adding a dialog using the new tag can be achieved with just a few lines of code. ` However, adding the dialog alone won’t do anything to the page. It will show up only once you call the .showModal() method against it. ` Then if you want to close it you can call the .close() method on the dialog, or press the escape key to close it, just like most other modals work. Also, note how a backdrop appears that darkens the rest of the page and prevents you from interacting with it. Neat! Accessibility and focus management Correctly handling focus is important when making your web applications accessible to all users. Typically you have to move the current focus to the active dialog when showing them, but with the dialog element that’s done for you. By default, the focus will be set on the first focusable element in the dialog. You can optionally change which element receives focus first by setting the autofocus attribute on the element you want the focus to start on, as seen in the previous example where that attribute was added to the close element. Using the .showModal() method to open the dialog also implicitly adds the dialog ARIA role to the dialog element. This helps screen readers understand that a modal has appeared and the screen so it can act accordingly. Adding forms to dialogs Forms can also be added to dialogs, and there’s even a special method value for them. If you add a element with the method set to dialog then the form will have some different behaviors that differ from the standard get and post form methods. First off, no external HTTP request will be made with this new method. What will happen instead is that when the form gets submitted, the returnValue property on the form element will be set to the value of the submit button in the form. So given this example form: ` The form element with the example-form id will have its returnValue set to Submit. In addition to that, the dialog will close immediately after the submit event is done being handled, though not before automatic form validation is done. If this fails then the invalid event will be emitted. You may have already noticed one caveat to all of this. You might not want the form to close automatically when the submit handler is done running. If you perform an asynchronous request with an API or server you may want to wait for a response and show any errors that occur before dismissing the dialog. In this case, you can call event.preventDefault() in the submit event listener like so: ` Once your desired response comes back from the server, you can close it manually by using the .close() method on the dialog. Enhancing the backdrop The backdrop behind the dialog is a mostly translucent gray background by default. However, that backdrop is fully customizable using the ::backdrop pseudo-element. With it, you can set a background-color to any value you want, including gradients, images, etc. You may also want to make clicking the backdrop dismiss the modal, as this is a commonly implemented feature of them. By default, the <dialog> element doesn’t do this for us. There are a couple of changes that we can make to the dialog to get this working. First, an event listener is needed so that we know when the user clicks away from the dialog. ` Alone this event listener looks strange. It appears to dismiss the dialog whenever the dialog is clicked, not the backdrop. 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The dialog element itself also has some default padding that will exacerbate this issue. ` The wrapping div can be made into an inline-block element to contain the margin, and by moving the padding from the parent dialog to the wrapper, clicks made in the padded portions of the dialog will now interact with the wrapper element instead ensuring it won’t be dismissed. Conclusion Using the dialog element offers significant advantages for creating dialogs and modals by simplifying implementation with reasonable default behavior, enhancing accessibility for users that need assistive technologies such as screen readers by using automatic ARIA role assignment, tailored support for form elements, and flexible styling options....

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Understanding the Difference Between `:focus` and `:focus-visible` in CSS

Understanding the Difference Between :focus and :focus-visible in CSS I have learned my fair share about the importance of keyboard accessibility, so I know that visual indication of the focused element is very important. But the well-known :focus pseudo-class is not always the best fit for this job. That's where :focus-visible comes in. Let's look at the differences between these two pseudo-classes and explore the best practices for using them effectively. What is the :focus Pseudo-Class? The :focus pseudo-class is a CSS selector that applies styles to any element that receives focus, regardless of how that focus was triggered. This includes focus events from keyboard navigation, mouse clicks, and touch interactions. Example Usage of :focus ` In this example, the button will display a blue outline whenever it is focused, whether the user clicks on it with a mouse, taps it on a touchscreen, or navigates to it using the keyboard. What is the :focus-visible Pseudo-Class? The :focus-visible pseudo-class is more specialized. It only applies styles to an element when the browser determines that the focus should be visible. This typically occurs when the user navigates via the keyboard or assistive technologies rather than through mouse or touch input. Example Usage of :focus-visible ` Here, the button will only show a blue outline when focused through keyboard navigation or another input method that usually requires visible focus indicators. Key Differences Between :focus and :focus-visible :focus - Behavior: Applies to any element that receives focus, regardless of the input method. - Use Cases: Ensures that all interactions with the element are visually indicated, whether by mouse, keyboard, or touch. :focus-visible - Behavior: Applies styles only when the focus should be visible, such as using a keyboard or assistive technology. - Use Cases: Ideal for scenarios where you want to provide focus indicators only to keyboard and assistive technology users while avoiding unnecessary outlines for mouse and touch users, typically required by design. Accessibility Implications :focus - Pros: - Guarantees that all users can see when an element is focused, which is critical for accessibility. - Cons: - Can lead to a suboptimal experience for mouse users, as focus styles may appear unnecessarily during mouse interactions. :focus-visible - Pros: - Enhances user experience by showing focus indicators only when necessary, thus keeping the interface clean for mouse and touch users. - Tailors the experience for keyboard and assistive technology users, providing them with clear visual cues. - Cons: - Additional considerations may be required to ensure that focus indicators are not accidentally omitted, especially in older browsers that do not support :focus-visible. - There may be cases where you want to show focus indicators for all users, regardless of input method. Best Practices for Using :focus and :focus-visible To achieve the best accessibility and user experience, combining both :focus and :focus-visible in your CSS is often a good idea. Combining :focus and :focus-visible ` Here is a Stackblitz example of what such styling could look like for you to try out and play with. Additional Tips - Test with Keyboard and Assistive Technology: Ensure that your web application is navigable using a keyboard (Tab, Shift + Tab, etc.) and that focus indicators are visible for those who rely on them. It's never a bad idea to include accessibility testing in your e2e testing suite. - Provide Clear Focus Indicators: Make sure that focus indicators are prominent and easy to see. A subtle or hard-to-spot focus indicator can severely impact accessibility for users who rely on keyboard navigation. Conclusion The :focus-visible pseudo-class offers a more refined way to manage focus indicators, improving accessibility and user experience, particularly for keyboard and assistive technology users. By understanding the differences between :focus and :focus-visible, and applying best practices in your CSS, you can create more accessible and user-friendly web applications. Remember, accessibility should never be an afterthought. By thoughtfully applying focus styles, you ensure that all users, regardless of how they interact with your site, can easily navigate and interact....

Nov 8, 2024

3 mins

AccessibilityCSSPlaywright

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New Core Web Vitals and How They Work

What are Core Web Vitals?

What's Changing in March 2024?

Why the Change?

How Does INP Work?

How to Prepare for INP?

1. Measure

2. Do not panic

3. Optimize

Conclusion

Jan Kaiser

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