Every performance term

.htaccess A per-folder Apache settings file used here for compression and caching rules.
304 Not Modified A tiny server reply saying the browser's copy is still current.
75th percentile The visit three quarters of the way from fastest to slowest; Google judges vitals there.
<noscript> Content used only when JavaScript is off, such as a plain stylesheet link.
<picture> An element that offers several image files and lets the browser pick one.
<source> A child of `<picture>` that offers one file, with `type` or `media` to say when to use it.
@font-face A CSS rule that names a web font family and where its file lives.
@import A CSS rule that loads another stylesheet, found only once the first one arrives.
AddOutputFilterByType An Apache directive that applies a filter, such as compression, to chosen file types.
art direction Using `<picture>` with `media` to show a different crop on different screens.
as attribute Tells a preload what kind of file it is, so it matches the later request.
aspect-ratio A CSS property that sets an element's shape, such as `3/2`, before content arrives.
async Downloads a script in the background and runs it as soon as it arrives, in any order.
AVIF A newer image format that often beats WebP on photo size, slower to encode.
background-image A CSS image the preload scanner cannot see until the CSS is applied.
Brotli A newer compression method that usually beats gzip on text, `BROTLI_COMPRESS` in Apache.
browser cache The browser's store of recent files, reused while they are still fresh.
bundle Several scripts joined into one file by a build tool.
cache busting Changing a file's URL when its contents change so browsers fetch the new one.
Cache-Control The response header that tells the browser whether and how long to keep a file.
Chrome DevTools The developer tools built into Chrome, opened with F12.
Chrome UX Report Google's collection of real Chrome users' speed data over the past 28 days.
CLS (Cumulative Layout Shift) A score for how much visible content moved without the visitor asking.
compression Squeezing a file on the server so fewer bytes travel, then unpacking it in the browser.
connection setup Looking up a server, opening a connection and agreeing on encryption before a download.
Content-Encoding The response header that says how a file was compressed, like `br` or `gzip`.
Core Web Vitals Google's three page experience numbers: LCP, CLS and INP.
critical CSS The few rules needed to draw the first screen, placed inline in the page.
crossorigin An attribute needed on font preloads and preconnects because fonts use CORS requests.
CSS pixel The unit layout uses; a 2x screen draws each one with four real pixels.
currentSrc An image property showing which file from `srcset` the browser chose.
defer Downloads a script in the background and runs it in page order after the HTML is read.
device pixel ratio How many real screen pixels a phone uses for each CSS pixel, often 2 or 3.
DOMContentLoaded The event fired once the HTML has been fully read and deferred scripts have run.
facade A light stand-in, such as a button, that loads a heavy widget only when used.
fallback font The font used while the web font is missing, ideally similar in width.
FCP (First Contentful Paint) When anything at all first appeared on screen.
fetchpriority="high" Tells the browser a file is urgent from the moment it reads the tag.
field data Numbers collected from real visitors on their own devices and networks.
FilesMatch An Apache block that applies rules only to files whose names match a pattern.
filmstrip A row of screenshots taken as the page loads, useful for spotting jumps.
fingerprinted file name A name with a short code from the file's contents, like `shop.3f9a1c.css`.
first and second visit A test with an empty cache compared with one where the browser kept files.
First Input Delay An older measure of tap delay that INP replaced as a Core Web Vital in 2024.
font-display An `@font-face` setting that decides what text does while the font is missing.
font-display: swap Shows a fallback font at once, then switches to the web font when it lands.
good, needs improvement, poor The three bands each vital falls into, judged at the 75th percentile.
gzip A widely supported compression method, turned on in Apache with `DEFLATE`.
height: auto A CSS rule that lets an image's height follow its width so it keeps its shape.
hero image The large photo at the top of a page, often the LCP element.
HTTP/2 A protocol that sends many files at once over one connection, making request count matter less.
image quality setting An export number, usually 0 to 100, for how much detail a lossy format may drop.
immutable A `Cache-Control` value promising the file will never change, so no recheck on reload.
inline <style> CSS written inside the HTML, which arrives with the page and needs no request.
inline SVG A drawing written into the HTML, which needs no extra request and scales cleanly.
INP (Interaction to Next Paint) How long a tap, click or key press takes to show its result on screen.
input delay The part of INP spent waiting for the main thread to be free after a tap.
intrinsic size The real pixel width and height of an image file.
lab data Numbers from a test you run under fixed, repeatable conditions.
layout shift Visible content moving on its own while the page loads or changes.
LCP (Largest Contentful Paint) When the biggest image or text block on the first screen finished painting.
LCP element The image or text block whose paint time counts as the page's LCP.
Lighthouse Chrome's built-in lab test that scores a page and lists suggestions.
Lighthouse CI Runs Lighthouse on each change and can fail the build when a budget is broken.
load on interaction Running a feature's code only when someone asks for it, instead of at load.
loading="lazy" Lets the browser wait to fetch an image or iframe until it nears the screen.
long task Any single stretch of main-thread work over 50 ms.
lossy format An image format that throws away some detail to save bytes, like JPEG, WebP or AVIF.
main thread The one lane that runs scripts and also handles taps, scrolling and drawing.
max-age A `Cache-Control` value giving how many seconds a copy stays fresh.
media attribute (on link) Tells the browser when a stylesheet applies, so non-matching sheets do not block paint.
media="print" Marks a stylesheet as print only, so screens paint without waiting for it.
min-height A CSS property that reserves space for content a script will add later.
Network panel The DevTools panel listing every request with its size and timing.
network throttling Making a fast computer pretend to be on a slower connection for testing.
no-cache Lets the browser keep a file but makes it check with the server before each use.
no-store Tells the browser never to keep the file at all.
onload media swap Loading a sheet as print, then switching it to `all` once it arrives.
origin A site's scheme, name and port; each new one needs its own connection.
page weight The total kilobytes a page downloads.
PageSpeed Insights Google's web tool that shows real-visitor data and a Lighthouse lab test for a URL.
parser-blocking Describes a script that stops the HTML being read until it downloads and runs.
performance budget A short list of limits, such as KB, requests and LCP, a page must stay inside.
Performance panel The DevTools panel that records a load and shows main-thread work and long tasks.
position: fixed Floats an element over the page, so adding it moves nothing else.
preconnect A `<link rel="preconnect">` that opens a connection to another server ahead of time.
preload A `<link rel="preload">` that fetches a needed file early at high priority.
preload scanner A quick second reader that skims ahead in the HTML to start downloads early.
processing time The part of INP spent running your event handlers.
render-blocking Describes a file the browser must get before it can paint anything.
request chain Files that are only found after another file arrives, each adding a wait.
request priority How the browser shares one network link between files, giving urgent ones more.
resource hint A `<link>` that tells the browser early about a file or server it will need.
round trip One message to the server and its reply; a new connection needs several.
scheduler.yield() Ends the current task so the browser can paint and handle taps, then carries on.
scroll snap A CSS feature that makes a row of items swipe and settle without a script.
sizes An attribute telling the browser how wide an image will show, in CSS pixels.
srcset An `<img>` attribute listing several sizes of the same picture for the browser to choose from.
system font stack A font list like `system-ui, sans-serif` that uses the device's own font and costs nothing.
TBT (Total Blocking Time) A lab number adding up how long long tasks kept the main thread busy past 50 ms.
third-party script Code loaded from another company's server, such as a chat widget or map.
time to first byte How long the browser waits for the server's first byte after asking.
type attribute (on source) Names a source's file format so the browser can skip ones it cannot show.
w descriptor A note in `srcset`, like `800w`, giving a file's real width in pixels.
waterfall A chart with one bar per request on a shared timeline, showing when each started and ended.
web font A font file the page downloads instead of using one on the device.
WebP A modern image format usually smaller than JPEG, supported by every current major browser.
width and height attributes Numbers on an `<img>` that give its shape so space is kept before it loads.
WOFF2 The compressed font file format used for web fonts.
x descriptor A note in `srcset`, like `2x`, for fixed-size images picked by pixel ratio.