Transparency support is table stakes for both PNG and WebP, so "does it support transparency" isn't the decision point anymore. What actually differs is how each format compresses transparent regions, how tooling handles the conversion, and where the edge cases show up.
Both Have a Real 8-Bit Alpha Channel
PNG's alpha channel is defined per the PNG specification as an 8-bit value per pixel when using the RGBA color type, giving 256 levels of transparency. WebP's container format also supports a full 8-bit alpha channel. Neither format is limited to a hard on/off transparency like GIF — both handle smooth alpha gradients (soft shadows, feathered edges, glass effects) at full precision. If you've seen claims that one format has "better" transparency in terms of bit depth, that's not accurate for a PNG-vs-WebP comparison; the difference is elsewhere.
Where the Real Difference Lives: Compression of the Alpha Channel
PNG
PNG compresses the alpha channel using the same filter-then-DEFLATE pipeline it uses for color channels — lossless, deterministic, and reasonably efficient for alpha data that's mostly flat (fully opaque or fully transparent regions) with occasional soft edges.
WebP lossless
WebP's lossless mode uses more advanced prediction and entropy coding than PNG's DEFLATE, and in most real-world transparent graphics — icons, logos, UI elements — produces meaningfully smaller files than an equivalent lossless PNG while preserving the alpha channel exactly. This is the closest to an apples-to-apples upgrade: same guarantee of pixel-perfect output, generally smaller file.
WebP lossy
WebP's lossy mode compresses the alpha channel with its own quantization pass, separate from the RGB channels. This is where behavior diverges from PNG in a way that matters for content choice: lossy alpha compression can soften hard transparency edges slightly — a crisp cutout boundary (think a logo silhouette against transparency) may show minor edge blur under aggressive lossy settings, while a photographic cutout with naturally soft edges (a portrait with hair detail) tends to hide this artifact well since the edge was never perfectly hard to begin with.
Decision Point: Which Mode for Which Content
Icons, logos, UI graphics with hard alpha edges: use PNG or lossless WebP. The precision matters here because viewers notice edge softening on geometric shapes and text far more readily than on organic content.
Photographic cutouts, soft shadows, gradient transparency: lossy WebP is usually safe and meaningfully smaller, since the softness lossy compression might introduce is imperceptible against content that was already soft-edged.
Anything you'll composite or re-edit later: stay lossless (PNG or lossless WebP) until final export, for the same generation-loss reason that applies to any lossy format in an editing pipeline.
# Compare lossless WebP against lossy WebP alpha handling on your own asset
cwebp -lossless input.png -o lossless.webp
cwebp -q 80 input.png -o lossy.webp
ls -la input.png lossless.webp lossy.webp
Check both outputs at 100% zoom on the alpha edges specifically — not just the overall image — since that's where the two modes diverge.
Tooling and Compatibility Gaps
PNG transparency support is universal across every image tool, browser, OS thumbnail renderer, and design app that's existed for the last two decades. WebP transparency support is broad in modern browsers — see caniuse.com's WebP coverage — but gaps still show up in older image editors, some email clients, and third-party tools that added WebP support later than PNG support. If a transparent image needs to render correctly somewhere you don't fully control — a Slack unfurl, an email template, a PDF export pipeline — verify WebP support in that specific consumer before relying on it, or keep a PNG fallback.
A Practical Comparison
| Aspect | PNG | WebP (lossless) | WebP (lossy) | | --- | --- | --- | --- | | Alpha bit depth | 8-bit | 8-bit | 8-bit | | Alpha compression | DEFLATE | Advanced entropy coding | Quantized, separate from RGB | | Hard-edge fidelity | Exact | Exact | May soften slightly | | Typical file size | Baseline | Usually smaller than PNG | Smallest | | Universal tool support | Yes | Broad, not universal | Broad, not universal | | Safe for re-editing | Yes | Yes | No (lossy) |
Converting Without Breaking Transparency
The most common mistake when converting formats is not checking that transparency survived the round trip — some converters flatten alpha to a solid background by default if not explicitly configured to preserve it. When moving a transparent asset between formats, always verify the output against a checkered or colored background, not just white, since a fully-opaque white background can silently mask a transparency loss.
Use PNG to WebP when you want the smaller lossless WebP output for icons and UI graphics, and WebP to PNG when you need to get back to universally-compatible PNG for a consumer that doesn't reliably support WebP. For vector-based transparent graphics like icons and logos, also consider whether the asset should be SVG instead — a vector format sidesteps the raster alpha-compression question entirely for shapes that don't need photographic detail.
Next Step
Don't assume either format is strictly better for transparency — test your specific asset type. Run a hard-edge graphic and a soft-edge photo through both PNG and WebP lossless/lossy modes, compare file size against visible edge quality at the resolution you'll actually display it, and pick per asset type rather than adopting a blanket rule across your whole image library.