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No published test behind this headline supports a 10x winner. The article has no image set, settings, encoder details or byte counts to check, so treat the 10x figure as unverified. What the current web guidance does establish is how each lever works, what it can and cannot do, and how you can run a fair comparison on your own images. Resizing comes first because it sets the pixel count everything else acts on, but it is not guaranteed to be the largest saving on every page.
What the 10x headline can and cannot support
A claim that one image lever beats the others by ten times would need the starting files, the crop, the rendered size, the format and encoder settings, the quality target, and the byte counts before and after each change. None of that is published alongside the headline, so readers cannot reproduce it or know which kind of image it applies to. The three levers are real and documented, but their relative savings depend heavily on the image. A large photo served at three times its display size can shrink dramatically when you correct its dimensions. A flat logo may gain little from a format change and much from being served as a vector. Use the 10x figure as a hypothesis to test, not as a rule.
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Lever 1: Resize to the rendered size
web.dev’s Image performance guide puts the priority plainly:
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Source: web.dev, Image performance.
Measure the rendered size at each breakpoint
- Load the page at each layout width you support, such as a 400 px phone, a 800 px tablet and a 1200 px desktop.
- Use your browser’s developer tools to read the rendered width and height of each image in CSS pixels.
- Record the largest rendered width per breakpoint. That number, not the source file’s dimensions, is the size you need to serve.
- Check the device pixel ratio separately. A 2x or 3x screen needs more pixels for the same CSS size, which is why a file sized to its CSS width can still look soft on high-density displays.
Provide several candidate files with srcset and sizes
For responsive pages, srcset lists candidate files and sizes describes how wide the image will be rendered. The browser then chooses the candidate that fits the layout and pixel density.
<img src="hero-800.jpg"
srcset="hero-400.jpg 400w, hero-800.jpg 800w, hero-1200.jpg 1200w"
sizes="(max-width: 600px) 100vw, 800px"
width="800" height="533"
alt="Ceramic mug on a wooden table">Each extra candidate adds files to store and cache, and more markup to maintain. Two or three well-chosen widths usually cover typical layouts; more variants are justified only when your layouts differ sharply.
Keep width and height so space is reserved
Declare the image’s intrinsic width and height (or an equivalent aspect ratio in CSS). The browser uses these to reserve the correct box before the file loads, which reduces layout shift. Resizing the file does not replace this step; it complements it.
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The same CSS size needs more device pixels on a high-density screen. web.dev illustrates this with uncompressed pixel counts for an image shown at 100×100 CSS pixels. These are illustrations of pixel density, not measurements of compressed files.
| Pixel density | Device pixels needed for 100×100 CSS px | Uncompressed size (as stated by web.dev) |
|---|---|---|
| 1x | 100×100 = 10,000 px | 40,000 bytes |
| 2x | 200×200 = 40,000 px | 160,000 bytes |
| 3x | 300×300 = 90,000 px | 360,000 bytes |
Source: web.dev, Choose the right image format. The page does not state a publication date for these figures.
Lever 2: Choose the format for the content
Format choice matters most when the content type is clear. Photographs and continuous-tone images fit raster formats. Geometric illustrations, icons and diagrams are often better as vector files. MDN describes WebP and AVIF as generally better at compressing raster images than older formats, but support and features still vary, so both usually need a fallback.
Rank #3
| Format | Typical content | Strengths noted in the guidance | Watch for |
|---|---|---|---|
| JPEG | Photographs | Long-established lossy format and a common baseline for comparison | Lossy compression discards detail; artifacts show on text and sharp edges |
| PNG | Graphics needing exact pixels or transparency | Lossless, so no quality loss from compression | Often larger than lossy formats for photographs |
| WebP | Photographs and graphics | MDN reports an average of 25–35% smaller than JPEG at visually similar lossy compression levels (MDN Web Docs, publication date not stated on the page) | Average, not a guarantee for every image; check support for your audience |
| AVIF | Photographs and graphics | MDN describes it as generally better-compressing than older formats | Support and encoding time vary; test before relying on it |
| SVG | Icons, logos, diagrams, geometric illustrations | Scales to any size without a new raster file | Not suited to photographs |
Source: MDN Web Docs, Image file type and format guide. Also see MDN’s Multimedia: Images guide.
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Serve a modern format with a fallback
A picture element lets the browser choose the first source format it supports and falls back to a JPEG otherwise.
<picture>
<source type="image/avif" srcset="hero-800.avif">
<source type="image/webp" srcset="hero-800.webp">
<img src="hero-800.jpg" width="800" height="533" alt="Ceramic mug on a wooden table">
</picture>Check current browser compatibility data before publishing a site-wide decision, since support changes over time.
Lever 3: Tune quality on the actual image
Lossy compression can reduce bytes substantially, but it removes information that cannot be recovered. Photographs often hide that loss well. Text, thin lines, logos and screenshots show it quickly. web.dev notes there is no universal quality setting: compare several compression levels and inspect each result at the size it will be displayed.
- Compare at least three settings on each image type you serve, not one setting for the whole site.
- View each output at 100% and at the displayed CSS size, on a standard screen and a high-density screen.
- Check edges and small text first. If lettering or hairlines blur, raise the quality setting, switch to a lossless format, or use SVG for diagrams.
- Record the setting that gives the smallest file you would accept for each content type, rather than a single number that you apply everywhere.
How to run a fair comparison on your own images
If you want to test which lever matters most for your site, keep the comparison reproducible:
- Start from the same source images and the same crop.
- Report the pixel dimensions of the source and the rendered dimensions on the page.
- Name the format, the encoder and its version, and every setting used.
- State the quality target and how you judged it, for example by visual review at display size.
- Label each output as lossy or lossless.
- Report bytes and percentage reduction against one declared baseline, such as the original JPEG at its full size.
- Test photographs and graphics separately, because they respond to the levers differently.
- Note fallback needs and delivery effects, since a second format doubles the files you store.
This checklist is a practical method drawn from the documented trade-offs. It is not an official benchmark protocol.
Best Value
Where automated delivery fits
An image transformation service can handle resizing, format selection and delivery automatically. That helps image-heavy sites that generate many sizes. It also adds a dependency and a cost. Compare sizing controls, format support, cache behavior and pricing for a specific provider before adopting one. The browser-native steps above work without any third-party service.
The Bottom Line
Resize first, because every other lever acts on the pixels you keep. Choose the format by content type: a modern format with a fallback for photographs, SVG for diagrams and icons, and lossless output where exact pixels matter. Then set compression by inspecting each image type at its displayed size. Which lever saves the most depends on the image, so measure your own set with the method above before accepting any single multiplier, including the 10x figure in the headline.
Quick Recap
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