How do you diagnose a broken HTML report?

Open the browser console first. A 404 on `.css`/`.js` means missing assets, a CORS error against `file:///` means an opaque origin, 'Refused to load … Content Security Policy' means the host's policy, and 'requested an insecure resource' means mixed content. The message names the cause before any guessing starts.

Why do so many reports break only after they leave your machine?

Because the three things that make them work locally — the whole output directory, a permissive origin, and your network — all disappear in transit. A report is only really tested the first time someone else opens it.

What single change prevents most of these?

Publish the whole output directory to a URL rather than passing the entry file around. That fixes missing assets, the `file://` origin, artifact viewers with strict policies, and attachment size limits in one step.

Do interactive reports keep working when published?

Yes. Scripts run inside a sandboxed iframe (`allow-scripts`, no `allow-same-origin`), so charts, filters, sorting and drill-downs behave as they do locally.

Is there a size limit?

Entry HTML 5 MB; assets 25 MB per file, 250 MB and 500 files per report. Reports that exceed the file count are usually per-test JSON trees, where a single-file export mode is the escape hatch.

HTML report troubleshooting

A report that works on your machine and breaks everywhere else is not a random event. It is one of about six causes, and the browser console names which one before you start guessing.

Start here: read the console

Console says	Cause	Page
404 on `.css` / `.js`	Only the entry file travelled	Lost CSS
CORS error against `file:///…`	Opaque local origin	Allure blank · Playwright blank
`Not allowed to load local resource`	`file:///` path on a web page	Local resource blocked
`Refused to … violates … Content Security Policy`	The host's policy	Mixed content vs CSP
`…requested an insecure resource…`	Mixed content	Mixed content
`ERR_FILE_NOT_FOUND` on a local link	File or sibling page isn't there	File not found
`Ã©` / `â€™` where accents should be	Wrong encoding guessed	Weird characters
Nothing — the page just downloads	Wrong `Content-Type`	S3 downloads instead of rendering
Nothing — the chart box is empty	The library never loaded	Plotly missing
`can't acquire context` / `Chart is not defined`	Script order or missing library	Chart.js not showing
`attribute d: Expected number, "MNaN…"`	A D3 scale got strings or undefined	D3 not showing
`Initialize failed: invalid dom` / empty chart div	ECharts container missing or 0px tall	ECharts not showing

Blank pages and missing content

Allure report opens as a blank page — the XHR-loaded data/ tree, and why allure open works
Playwright report is blank or won't open — show-report, traces, and the CI zip that makes it worse
Plotly chart missing from the exported HTML — the four include_plotlyjs modes
Chart.js chart not showing — every console error matched to its fix, plus zero-height containers
D3 chart not showing — NaN paths, empty selections, and v4/v5/v6 breaking changes
ECharts chart not showing — zero-height containers, hidden tabs, echarts.use imports and unregistered maps
CORS error from origin 'null' — why a file:// page can't fetch its own folder or load modules
Jupyter widgets don't render in the export — what survives without a kernel
My HTML report lost its CSS — five causes, in order
The HTML file shows code instead of the page — extension, Content-Type, or a source-only viewer
Coverage report links 404 on click-through — the directory you left behind
ERR_FILE_NOT_FOUND in Chrome — moved files, zip previews, missing sibling pages, # in names

Hosting and delivery

GitHub Actions artifact HTML won't open — why it is always a zip
GitLab Pages report 404s — the public/ rule and branch collisions
The CI artifact link expired — four deletion mechanisms, only one of them retention
S3 downloads your HTML instead of rendering it — the Content-Type fix
Mixed content blocked — telling it apart from a CSP refusal
The localhost link doesn't work for others — why, and the two real options

Getting it in front of people

Images not showing in a sent HTML file — six path mistakes, and how to embed the images
“Not allowed to load local resource” — a file:/// path inside a web page
The HTML file is too big to email — what is making it large, and what to cut
Slack won't preview an HTML file — the security reason, and what to post
Excel “Save as Web Page” breaks when sent — the _files folder, and one file that works
CSV opens wrong in Excel — garbled accents, one column, lost leading zeros
â€™ and Ã© instead of quotes and accents — decoding the garbage, and the one tag that fixes it
The report is tiny on a phone — the viewport tag, and the fixed widths that survive it
Outlook blocked the HTML attachment — why phishing filters catch reports, and what gets through
Background colors missing when printing — the ink-saving default, and print-color-adjust
Table split across PDF pages — sliced rows, missing headers, clipped columns
The agent's output is too long to paste — comment and message ceilings

The pattern underneath all of them

Three things make a report work on your machine, and all three are lost in transit:

The whole output directory is present. Copy the entry file out and every relative reference breaks.
The origin is permissive. Your local server gave it a real http:// origin; file:// on the recipient's laptop does not.
Your network reaches the CDN. Theirs might not, and the viewer they open it in may refuse the request regardless.

Publishing the output directory to a URL restores all three at once: the entry HTML becomes the report body, assets upload alongside it with their relative references rewritten to the uploaded copies, and the whole thing is served over HTTPS from an address that does not depend on anyone's machine being awake.

Interactivity survives — scripts run in a sandboxed iframe (allow-scripts, no allow-same-origin) — and readers can select the failing assertion or the suspicious number and comment on it, rather than screenshotting it into chat (comment on HTML).

Limits: entry HTML 5 MB; assets 25 MB per file, 250 MB and 500 files per report.

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