70 terms
Glossary
The terms used across the experiments, in plain language. Each links to where you can see it happen.
No terms match that.
#
- 103 Early Hints
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An HTTP response a server can send before the real one, listing resources to preload or connect to, so the browser can start while the server is still working.
A
- async and defer
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Attributes on a script tag. Both download the script without stopping the HTML parser. defer runs it after parsing, in order; async runs it as soon as it arrives, in any order. Neither makes the script cheaper to run.
- AVIF
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An image format based on a video codec. Often the smallest for photographs at a given quality, with slower encoding. Supported by current browsers.
B
- Back/forward cache bfcache
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A browser feature that keeps a whole page, JavaScript state included, in memory when you leave it, so back and forward restore it in milliseconds. An unload handler (on desktop Chrome and Firefox) and a no-store response are among the things that can disqualify a page.
- Bandwidth
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How many bytes per second a connection can carry. It limits how fast a large file downloads, but not how long a small one waits for a round trip.
- Block period
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The time at the start of font loading during which text using that font is invisible. font-display sets its length.
- Brotli
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A compression format that usually beats gzip for text such as HTML, CSS and JavaScript. Supported by all current browsers over HTTPS.
- Bundle
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Several source files combined into one for delivery. Fewer requests, but a change to any file invalidates the whole thing in the cache.
C
- Cache-Control
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The HTTP header that tells a cache how long a response is fresh and what it may do with it: max-age, no-cache, no-store, immutable, stale-while-revalidate and others.
- CDN content delivery network
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A network of servers close to visitors that cache your responses, shortening the round trip. Also called an edge cache.
- Code splitting
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Breaking JavaScript into pieces so a page loads only the code it needs now and fetches the rest later.
- Compositing composite
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The last rendering step: painted layers are combined and sent to the screen, largely off the main thread. Animating transform and opacity usually needs only this step.
- Compression Content-Encoding
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Shrinking a response for transfer (gzip, Brotli, Zstandard). It shortens the download, not the work of parsing the decompressed file.
- content-visibility
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A CSS property. With auto, the browser skips rendering work for off-screen elements until they approach the viewport.
- Core Web Vitals
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Google’s three field metrics for user experience: LCP (loading), INP (responsiveness) and CLS (visual stability), judged at the 75th percentile of real visits.
- Critical CSS
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The small amount of CSS needed for the first view, inlined in the HTML so the rest can load without blocking rendering.
- Critical rendering path
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The sequence the browser must complete before it can paint: HTML to DOM, CSS to CSSOM, render tree, layout, paint, composite.
- CSSOM
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The CSS Object Model: the browser’s parsed structure of every style rule. The render tree can’t be built until it’s complete.
- Cumulative Layout Shift CLS
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A Core Web Vital: a score for how much visible content moves unexpectedly while the page loads and is used. Good is 0.1 or less; poor is above 0.25.
D
- DNS lookup DNS
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Turning a hostname into an IP address. Costs about a round trip the first time, and is often cached after that.
- dns-prefetch
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A resource hint that resolves a hostname early. Cheap, and saves one round trip.
- DOM Document Object Model
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The tree of elements the browser builds from your HTML. It is built incrementally as the HTML arrives.
E
- ETag validator
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A fingerprint of a response. The browser sends it back to ask “has this changed?”, and the server can answer 304 Not Modified with no body.
F
- fetchpriority
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An attribute that hints a request is more (high) or less (low) important than the browser would assume. It reorders requests; it does not add bandwidth.
- Field data RUM, real-user monitoring
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Measurements from real visitors on their own devices and networks. It shows the whole distribution, including the slow tail.
- First Contentful Paint FCP
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The moment the first text or image is painted. A lab and field metric for how soon something appears.
- FOIT flash of invisible text
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Text is invisible while a web font loads, so the page looks loaded but unreadable.
- font-display
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A CSS descriptor that says how to behave while a web font loads: hide the text for a while (block), show a fallback at once (swap), or give up on the font (optional).
- FOUT flash of unstyled text
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Text appears in a fallback font and then changes when the web font arrives, often moving the layout.
G
- gzip
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The universal compression format for web text. Supported everywhere.
H
- HTTP/1.1
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A protocol where each connection carries one request at a time, so browsers open several connections per origin (typically six).
- HTTP/2
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A protocol that multiplexes many requests over one connection. A lost packet can stall every stream because they share a TCP connection.
- HTTP/3
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HTTP over QUIC, which runs on UDP. It multiplexes like HTTP/2, but packet loss affects only the stream it hit, and setting up a connection takes one round trip less.
I
- immutable
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A Cache-Control value that promises a file will never change at this URL, so the browser needn’t revalidate it. Used with fingerprinted filenames.
- Interaction to Next Paint INP
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A Core Web Vital for responsiveness: how long after a tap, click or keypress the next frame is drawn. Good is 200 ms or less; poor is above 500 ms.
L
- Lab data synthetic testing
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A measurement taken under fixed, controlled conditions, such as a Lighthouse run. Repeatable, but it describes only those conditions.
- Largest Contentful Paint LCP
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A Core Web Vital: when the largest image or text block in the viewport is painted. Good is 2.5 s or less; poor is above 4 s.
- Layout reflow
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Working out the size and position of every box. Changing something that affects size or position can force it to run again.
- Lazy loading
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Deferring a resource, usually an image, until it is near the viewport. Right below the fold; wrong for the largest image on screen.
- Long task
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Any main-thread task longer than 50 ms. The browser can’t interrupt it, so input arriving meanwhile has to wait.
M
- Main thread
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The single thread that runs your JavaScript, handles input, calculates styles and layout, and drives painting. While it’s busy, the page can’t respond.
- Minification
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Removing whitespace and shortening names in code without changing what it does. It shrinks both the download and what the browser has to parse.
- modulepreload
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A resource hint like preload for JavaScript modules: it also fetches and parses the module, so a chain of imports can be discovered at once.
- Multiplexing
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Sending many requests and responses interleaved over a single connection. Introduced by HTTP/2.
O
- Origin
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The combination of scheme, host and port. Each origin needs its own connection, with its own DNS lookup, TCP and TLS setup.
P
- Paint
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Turning boxes into pixels: backgrounds, text, borders, images.
- Parse and compile
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Reading source text and turning it into something the browser can run or apply. Costs main-thread time, in proportion to the uncompressed size.
- Percentile p75, p95
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The value below which a given share of visits fall. The 75th percentile means three in four visits were at least this good. It describes a skewed distribution better than the average does.
- preconnect
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A resource hint that does the whole connection setup (DNS, TCP and TLS) early. For origins you’re certain to use.
- prefetch
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Fetching something at the lowest priority for a likely next navigation. Speculation Rules prefetch fetches only the next page’s document.
- preload
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A resource hint that fetches a file now, at a priority set by its type, for use on this page. It helps where the browser would discover the file late.
- prerender
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Loading and rendering a likely next page in a hidden context so that visiting it is nearly instant.
R
- Render tree
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The DOM and CSSOM combined: the visible elements with their computed styles.
- Render-blocking
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A resource that stops the browser painting until it has loaded. Stylesheets in the head are render-blocking by default.
- Round trip RTT, latency
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The time for a message to reach the server and the reply to come back. Every new connection and every request pays at least one.
S
- Service worker
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A script that sits between a page and the network, can answer requests from a cache, and works when the visitor is offline. It has its own install and update lifecycle.
- Speculation Rules
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A JSON block on a page that tells the browser which links to prefetch or prerender, and how eagerly.
- srcset and sizes
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Attributes that let the browser choose among several sizes of an image. srcset lists the files and their widths; sizes says how wide the image will be displayed.
- stale-while-revalidate
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A Cache-Control extension that lets a cache serve a stale response immediately while it fetches a fresh one in the background.
- Subsetting
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Removing the characters you don’t need from a font file, for example keeping only Latin letters.
T
- TCP handshake TCP
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The exchange that opens a connection, taking a round trip before any data is sent. HTTP/3 doesn’t use TCP.
- Third-party script
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Code from a company that isn’t you: analytics, ads, chat, tag managers. You don’t control its size, speed or what it loads next.
- Time to First Byte TTFB, waiting
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How long after requesting something the first byte of the response arrives: one round trip plus the server’s own processing time.
- TLS handshake TLS, HTTPS
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The exchange that sets up encryption for a connection, costing about one more round trip with TLS 1.3.
- Total Blocking Time TBT
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A lab metric: the sum of every long task’s time beyond 50 ms between first paint and the page becoming interactive. A stand-in for INP in Lighthouse.
- Tree-shaking
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A build step that drops code that is never used from a bundle.
V
- Variable font
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One font file that holds a range of weights, widths or slants. Larger than one static weight and smaller than several.
- View transitions
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A browser feature for animating the change between two states or pages. It smooths a navigation; it doesn’t make it faster.