There are two fundamentally different approaches to comparing a Figma design against a live web page. Pixel-based comparison looks at what the page looks like. Spec-based comparison looks at what the page is made of. They catch different things, they miss different things, and they're useful at different stages.
Pixel-based tools take a screenshot of the design and a screenshot of the live page, then diff the two images. The output is a visual overlay or a heatmap showing where the pixels don't align.
This is the approach used by visual regression testing tools like Applitools, Percy, and Chromatic. It's also the principle behind overlay tools like PerfectPixel and Pixelay, where you manually layer the design on top of the browser and compare by eye.
The strength of pixel-based comparison is that it catches anything visible. A misaligned element, a colour that's slightly off, a font that didn't load, an image that's cropped differently. If it's visible on screen, a pixel diff can flag it.
The weakness is that it flags everything equally. A 1px shift from anti-aliasing gets the same weight as a 20px spacing error. Dynamic content (different product titles, varying text lengths) creates false positives. Responsive rendering differences between the design tool and the browser produce noise that isn't actionable. Pixel-based diffs tell you something is different. They don't tell you what property changed or by how much.
Spec-based tools read the design's properties (spacing, typography, colours, dimensions) directly from the Figma file, then read the same properties from the rendered web page's computed styles. The output is a list of specific differences: "font-weight is 400 in the implementation, 500 in the design" or "padding-top is 16px, design specifies 24px."
This approach compares the building blocks rather than the final image. It doesn't take screenshots. It reads the actual values from both sides and reports where they diverge.
The strength is precision. Every finding is a specific property with a specific delta. You know what to fix and by how much. There's no interpretation step. A designer or developer reading the output doesn't need to guess whether something is off. The report says exactly what's different.
The weakness is that spec-based comparison only catches what it measures. If a property isn't in the comparison set, the difference won't appear. Visual anomalies that don't map to a CSS property (a rendering glitch, a broken SVG, a transparency issue) won't show up in a spec-based report.
Overlay tools are a manual version of pixel-based comparison. You export the design as an image, load it in the browser, adjust the opacity, and compare by eye. PerfectPixel, Pixelay, and Over.fig all work this way.
The workflow is: export from Figma, import into the extension, position the overlay, toggle opacity back and forth, look for differences. It works. But the output is subjective. Two people looking at the same overlay might disagree on whether something is off. And the process has to be repeated every time something changes.
Overlay tools don't produce reports. They don't track changes across versions. They rely on the person doing the comparison to notice the differences and remember them.
A pixel-based diff catches visual regressions between two known states. It's strongest when comparing version A to version B of the same page, where the design hasn't changed and any pixel difference is an unintended regression. Visual regression testing tools are built for this exact use case, typically in CI/CD pipelines.
A spec-based comparison catches implementation accuracy. It's strongest when comparing a Figma design against a first implementation, or when checking whether a specific fix actually landed. The output answers: "does this element have the properties the design specifies?"
Neither approach catches everything. A pixel diff won't tell you the padding is 4px off. A spec comparison won't tell you the hero image is cropped wrong. Teams that care about both visual fidelity and implementation accuracy use both, at different points in their workflow.
If you're asking "did anything change since the last deploy," pixel-based regression testing is the right tool. The question is about change detection between two states.
If you're asking "does this implementation reflect the Figma design," spec-based comparison is the right tool. The question is about accuracy against a design source of truth.
If you're asking "does this look right," an overlay gives you a quick visual check. The question is subjective and the tool fits that.
Most teams default to overlays because they're familiar and free. The shift to either pixel-based or spec-based tooling happens when the team needs repeatability, shared reports, or integration into a review workflow.
The question isn't which approach is better. It's which question you're trying to answer. "Did something change" and "does this reflect the design" are different questions with different tools.
Uiprobe uses spec-based comparison: it reads properties directly from Figma and the live page, then reports the specific differences. No screenshots, no overlays, no manual positioning. Learn more about how Uiprobe compares Figma designs to live pages.
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