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Not as a universal rule. In one reported test on Chromium 149, a WebP created from a canvas at quality 1.0 decoded pixel-for-pixel to the source, but was about 3.5 times the size of the same test at 0.999. That is an observation about one browser build and image—not a guarantee that every browser treats 1.0 as lossless.
What the Canvas quality value does—and does not—promise
The Canvas API documentation describes the optional quality argument to HTMLCanvasElement.toBlob() as a number between 0 and 1 for formats that support lossy compression, including WebP. It does not define quality 1.0 as a cross-browser switch to lossless encoding.
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WebP itself supports both lossy and lossless encoding, as described in the WebP Container Specification and IETF RFC 9649. But the format’s capabilities do not tell you how a browser maps a particular numeric quality value to its encoder mode. Treat that mapping as something to verify in the browser versions your application supports.
What the reported 1.0 versus 0.999 test found
NullPointerZen reported a test of a 1440×2400 help-page screenshot on an Apple M4 laptop with 16 GB of memory, using a Chromium 149 open-source build. The WebP at quality 1.0 was 771,126 bytes; the file at 0.999 was 222,292 bytes—about 3.5 times smaller. The author also reported that all 13,824,000 channel values matched after decoding the quality-1.0 output, indicating a pixel-for-pixel match in that test. See the author’s account of the experiment.
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Those figures are one author-reported result, not a benchmark across browsers or a general prediction of WebP file sizes. The author said only Chromium 149 on that laptop was tested. Image content, browser implementation, and version can affect the result; the available evidence does not establish that 1.0 will always be lossless, or that it will always produce a file 3.5 times larger than 0.999.
How to check the output your application actually creates
If fidelity or format matters, test the actual images and browser builds you ship to. Record the returned Blob’s type and size; when losslessness is a requirement, decode the output and compare its pixels with the source. A file-size change alone cannot prove that the image is lossless.
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- Call
canvas.toBlob(callback, 'image/webp', quality)with the quality value you intend to use. - Inside the callback, check that the Blob is non-null, then record
blob.typeandblob.size. MDN documents that an unsupported requested type falls back to PNG, so inspect the returned type whenever WebP is required. - For a losslessness check, decode the Blob and compare its pixel values with the original canvas image data. Run the check on representative images and on each supported browser/version.
- If you search a range of quality values to meet a size limit, measure the output at the upper end in your target environment. A jump near 1.0 may make that endpoint a poor fit for a size-constrained search; it does not establish a universal cutoff.
Choosing a quality value in practice
Base the choice on the output mode and pixel fidelity you need, the byte size for your own image content, and the browser/version behavior you have verified. The reported test is a reason to check the upper endpoint rather than assume that moving from 0.999 to 1.0 makes only a negligible difference. It is not evidence for a general recommendation to cap quality at 0.99, nor does it prove what other browsers will do.
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