Image Format Checker

What the checker does

1. Reads the page's HTML once (up to 1.5 MB) and takes up to 20 images from <img> (with srcset and the common lazy-loading attributes), <picture> and og:image.
2. Requests each image twice: with Accept: image/avif,image/webp,…, as a current browser sends it, and with Accept: */*, as a client that asks for nothing in particular.
3. Reads the first bytes of each answer to tell the real format, whatever the file name or Content-Type says, and counts the bytes.
4. Reports the Vary header and the cache status the CDN printed, and follows up to 3 redirects, checking every target address again.
5. For a <picture> with an AVIF or WebP <source>, the modern request goes to that source and the other one to the <img>: there the browser chooses the format, not the server.

Reading the results

AVIF or WebP

A browser that supports these formats received one. The size next to it is what that browser downloaded; the size under "Other browsers" is what a browser without AVIF or WebP support gets.

Original only

The image came as JPEG, PNG or GIF even to a browser that asked for AVIF and WebP: the site does not convert it.

Same file for every browser

Both requests received the same format. That is the result for a site without format negotiation, and for a site that serves WebP files to everyone. With nothing to compare, no saving is shown.

Without Vary: Accept

The answer changes with the Accept header but does not say so with Vary: Accept, and shared caches are allowed to keep it. A proxy between the server and a visitor could then hand an AVIF or WebP file to a browser that did not ask for one.

The saving

Only images whose two answers were both read in full, and came in different formats, count toward the total. A size known only from Content-Length is shown but never added up: the figure is a measurement, not an estimate.

Rate-limited or blocked

When a site answers 429, 403, a bot check, or the small 503 page rate limiters send, the checker stops sending it requests for that check and marks the remaining images as not checked. Try again a few minutes later.

How a server chooses the format

A browser lists the image formats it can show in its Accept header. A server or CDN that converts images reads it and answers AVIF, WebP or the original from the same URL. It should then send Vary: Accept, so that caches keep one copy per format.

The other way is in the HTML: a <picture> element lists AVIF and WebP <source> files and the browser takes the first one it supports. Both work, and the checker follows whichever the page uses.

AVIF or WebP: which is smaller, and when

Limits and what we send

• Up to 20 images on up to 5 hosts per page, up to 5 MB read per image, 20 seconds per check.
• 3 checks a minute and 20 a day from one address. A finished check is reused for 10 minutes, and one site is checked at most 6 times in 10 minutes, whoever asks.
• Requests come from our server in Turkey over IPv4, with a User-Agent that ends in cdn.com.tr-image-check/1.0 and links to this page. A CDN may answer us from a different location than your visitors, some sites block automated clients, and a site reachable only over IPv6 cannot be checked.
• Only public addresses on ports 80 and 443 are requested; private and internal networks are refused. Images are not stored: only formats, sizes and a few headers come back.
• Images that JavaScript adds after the page loads, and CSS background images, are not in the HTML, so the checker does not see them.