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A Happo screenshot mismatch is evidence that the captured pixels changed—not proof that an Indian-language font is broken. Start by checking whether the diff repeats, then isolate font loading and fallback, text shaping, browser and viewport differences, and asynchronous page state. Adjust visual-diff tolerance only after you understand what changed.
What a Happo diff can—and cannot—tell you
Happo compares a captured page or component with a baseline. A highlighted difference tells you that the rendered image changed; it does not identify the cause. A mismatch may come from a genuine application change, a different capture state, a font or asset that was not ready, browser-specific rendering, or small rasterization noise.
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First open the before-and-after or diff view and locate the changed region. Classify what you see: different glyph shapes, missing characters, changed line breaks, altered baseline or line height, shifted element geometry, or broad low-level color noise. Record the browser target and viewport alongside the screenshot. Do not change the baseline or loosen comparison settings based only on a red diff indicator.
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- Reproduce the same capture. Retry the affected screenshot. If the difference moves or disappears, investigate timing, animations, and other non-deterministic page state before changing text or fonts.
- Match the inputs. Capture the same story or page state at the same viewport and browser target. If the failure may be browser-specific, repeat with another configured target.
- Verify font delivery and selection. Check the captured page’s network requests and computed styles. Confirm the intended font request succeeded, that the expected family and face were selected, and that the font was available before capture.
- Check character coverage and shaping. Verify that the selected face covers the exact characters in the failing sample. Compare the same text and CSS in a minimal reproduction, preserving combining marks and conjunct-forming sequences exactly.
- Stabilize the page. Check for late-loading fonts or images, animation, and other state that can vary between captures. Use Happo’s documented wait and animation controls where appropriate.
- Only then consider tolerance. A narrow color-delta tolerance may help with small anti-aliasing or compression differences. It is not a repair for missing glyphs, incorrect shaping, changed wrapping, or shifted layout.
Diagnose by the pattern of the diff
| What you observe | Checks to make | What it suggests |
|---|---|---|
| The result changes or vanishes on retry | Font and image loading, animation, other asynchronous page state | Capture instability is plausible; a single result may be spurious. |
| Only one browser target differs | Target configuration, browser-specific rendering, and font availability in that target | The issue may be specific to that browser environment. |
| Several targets differ in the same region | Shared page code, font assets, text content, or an intentional baseline change | A shared application or asset change is more likely than a single-target issue. |
| Characters are missing or replaced | Font request, selected fallback face, and glyph coverage for the sample | Check delivery and coverage before changing comparison settings. |
| Glyphs appear different or text wraps differently | Selected font, exact text sequence, shaping, viewport, and browser target | Treat it as a rendering or layout question, not automatically as pixel noise. |
| Many pixels show faint, low-amplitude noise | Whether the page content and geometry are otherwise stable | After review, a carefully limited color-delta tolerance may be relevant. |
Check font loading, fallback, and script coverage
Confirm the capture selected the intended face
In the page being captured, inspect the font request and the computed font-family and face. A stylesheet naming the desired family does not by itself establish that the corresponding file loaded or that the browser used it for every character. Check for a failed request, a late response, or fallback on only part of the text.
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Happo’s integration material describes waiting for fonts and asynchronous assets. That is useful protection, but it does not remove the need to verify your own font requests, CSS resolution, and capture setup when a particular glyph differs.
Verify coverage for the actual text
A font can load successfully and still lack one or more characters used in the sample. Compare the failing string character by character with the source text and confirm that the chosen face covers it. The relevant questions may involve OpenType support and shaping technologies such as HarfBuzz, as well as text processing associated with ICU. Their relevance makes them sensible areas to investigate; it does not establish that any one of them caused a particular Happo diff.
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Preserve the text sequence in a minimal reproduction
Reduce the failing component to the same text, font declarations, and relevant layout CSS. Check that the sample has not been normalized, escaped, truncated, or otherwise altered between the application and capture. Preserve combining marks and conjunct-forming sequences as they appear in the original string. Avoid inserting ad hoc spacing or replacing the language text just to make the baseline pass: that can hide the rendering problem instead of identifying it.
Control browser, viewport, and page state
For a useful comparison, keep the story or page state, viewport, and browser target consistent. A viewport change can alter wrapping and element geometry even when font rendering is unchanged. A different browser target can expose a target-specific rendering or font-environment difference. Happo’s documented targets include Chrome, Firefox, Edge, Safari, and iOS Safari, subject to the product’s current configuration.
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If only one target fails, focus on that target’s configuration and rendering environment. If the same area changes across targets, investigate shared code, assets, content, and baseline changes first. A local browser rendering is a useful diagnostic comparison, but it does not prove the Happo worker has identical font files or platform configuration.
Also look for changes in page state while the capture runs. Late-loading assets and animation can produce a mismatch that is not stable on retry. Happo documents controls for waiting on assets and silencing animations; use them to make the captured state comparable rather than masking differences in the screenshot.
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Adjust tolerance only for reviewed raster noise
Happo provides a color-delta tolerance for reducing minor differences such as anti-aliasing or compression noise. Apply it only after inspecting the diff and establishing that the content and layout are correct. Keep the threshold narrow and review the highlighted changes: a broad tolerance can conceal a real glyph, shaping, wrapping, or geometry regression.
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ScreenshotNeo is a screenshot API, not a replacement for Happo’s baseline comparison. It can provide a separate capture for inspecting a page or asset when you want a screenshot without setting up a browser script. Its API returns an image or PDF from one GET request; put your key in a secret or environment variable rather than publishing it.
cURL example, using the target page in this article:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://example.com -o shot.webpSee the ScreenshotNeo API documentation for parameters and response handling. ScreenshotNeo accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each step can be turned off. Bot checks and CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers report the page verdict and billing status. Its MCP server offers take_screenshot, get_page_info, and capture_pdf for AI agents and other MCP clients. The Free plan includes 1,000 shots per month without a card; paid plans start at $5 for 3,000 shots.
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Troubleshooting checklist
- Missing or substituted characters: inspect the font request, computed face, and coverage of the exact sample. Do not begin by raising diff tolerance.
- Text wraps differently: confirm viewport, font selection, text integrity, and shaping before treating the change as noise.
- Diff varies between retries: inspect asynchronous assets, font readiness, animation, and page state; use available wait or animation controls.
- Only one browser differs: reproduce with the same state and viewport in another configured target, then investigate the differing target’s environment.
- Every target changes similarly: check shared content, styles, assets, and whether the baseline should change because of an intentional UI update.
- Only faint pixel noise remains: review the actual region and consider a narrow color-delta tolerance, checking that meaningful text and layout changes remain visible.
Frequently Asked Questions
Does an Indian-language screenshot diff mean Happo cannot render that script?
No. A diff only establishes that captured pixels changed. The cause must be isolated from the screenshot and capture conditions; the available product and localization references do not establish a general Happo defect for Indian-language text.
Is a local screenshot enough to prove the Happo capture environment is correct?
No. It is a diagnostic comparison, not proof that the Happo worker uses identical browser configuration, platform settings, or font files.
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