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Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceEventTiming)
> [!WARNING]
> **Warning:** First Input Delay (FID) is no longer a Core Web Vital, and has been replaced by the [Interaction to Next Paint (INP) metric](https://web.dev/articles/inp.md.txt). As a consequence, [support for FID ended](https://web.dev/blog/inp-cwv-launch.md.txt) on September 9, 2024. **You should now focus on INP.**
We all know how important it is to make a good first impression. It's important
when meeting new people, and it's also important when building experiences on
the web.
On the web, a good first impression can make the difference between someone
becoming a loyal user or them leaving and never coming back. The question is,
what makes for a good impression, and how do you measure what kind of impression
you're likely making on your users?
On the web, first impressions can take a lot of different forms---we have
first impressions of a site's design and visual appeal as well as first
impressions of its speed and responsiveness.
While it is hard to measure how much users like a site's design with web APIs,
measuring its speed and responsiveness is not!
The first impression users have of how fast your site loads can be measured with
[First Contentful Paint (FCP)](https://web.dev/articles/fcp.md.txt). But how fast your site can paint pixels
to the screen is just part of the story. Equally important is how responsive
your site is when users try to interact with those pixels!
The First Input Delay (FID) metric helps measure your user's first impression of
your site's interactivity and responsiveness.
## What is FID?
FID measures the time from when a user first interacts with a page (that is, when
they click a link, tap on a button, or use a custom, JavaScript-powered control)
to the time when the browser is actually able to begin processing event handlers
in response to that interaction.
### What is a good FID score?
To provide a good user experience, sites should strive to have a First Input
Delay of **100 milliseconds** or less. To ensure you're hitting this target for
most of your users, a good threshold to measure is the **75th percentile** of
page loads, segmented across mobile and desktop devices.
> [!NOTE]
> **Note:** To learn more about the research and methodology behind this recommendation, see: [Defining the Core Web Vitals metrics thresholds](https://web.dev/articles/defining-core-web-vitals-thresholds.md.txt)
## FID in detail
As developers who write code that responds to events, we often assume our code
is going to be run immediately---as soon as the event happens. But as users,
we've all frequently experienced the opposite---we've loaded a web page on
our phone, tried to interact with it, and then been frustrated when nothing
happened.
In general, input delay (a.k.a. input latency) happens because the browser's
main thread is busy doing something else, so it can't (yet) respond to the user.
One common reason this might happen is the browser is busy parsing and executing
a large JavaScript file loaded by your app. While it's doing that, it can't run
any event listeners because the JavaScript it's loading might tell it to do
something else.
> [!TIP]
> FID only measures the "delay" in event processing. It does not measure the total event processing duration itself, nor the time it takes the browser to update the UI after running event handlers. While this time does affect the user experience, including it as part of FID would incentivize developers to respond to events asynchronously---which would improve the metric but likely make the experience worse. See [why only consider the input delay](https://web.dev/articles/fid.md.txt#why_only_consider_the_input_delay) below for more details.
Consider the following timeline of a typical web page load:
The above visualization shows a page that's making a couple of network requests
for resources (most likely CSS and JS files), and---after those resources
are finished downloading---they're processed on the main thread.
This results in periods where the main thread is momentarily busy, which is
indicated by the beige-colored
[task](https://html.spec.whatwg.org/multipage/webappapis.html#concept-task)
blocks.
Long first input delays typically occur between [First Contentful Paint
(FCP)](https://web.dev/articles/fcp) and [Time to Interactive (TTI)](https://web.dev/articles/tti.md.txt) because the page has
rendered some of its content but isn't yet reliably interactive. To illustrate
how this can happen, FCP and TTI have been added to the timeline:
You may have noticed that there's a fair amount of time (including three [long
tasks](https://web.dev/articles/custom-metrics#long-tasks-api)) between FCP and TTI, if a user tries to
interact with the page during that time (for example, by clicking on a link), there will be a
delay between when the click is received and when the main thread is able to
respond.
Consider what would happen if a user tried to interact with the page near the
beginning of the longest task:
Because the input occurs while the browser is in the middle of running a task,
it has to wait until the task completes before it can respond to the input. The
time it must wait is the FID value for this user on this page.
> [!NOTE]
> **Note:** In this example the user just happened to interact with the page at the beginning of the main thread's most busy period. If the user had interacted with the page just a moment earlier (during the idle period) the browser could have responded right away. This variance in input delay underscores the importance of looking at the distribution of FID values when reporting on the metric. You can read more about this in the section below on analyzing and reporting on FID data.
### What if an interaction doesn't have an event listener?
FID measures the delta between when an input event is received and when the main
thread is next idle. This means FID is measured **even in cases where an event
listener has not been registered.** The reason is because many user interactions
do not require an event listener but *do* require the main thread to be idle in
order to run.
For example, all of the following HTML elements need to wait for
in-progress tasks on the main thread to complete prior to responding to user
interactions:
- Text fields, checkboxes, and radio buttons (``, `