XML Validator — Well-Formedness Checker
Check XML well-formedness by validating tag matching, nesting, and angle bracket balance.
How it works
- 1Paste or type your text in the input field
- 2Click "Process" — processing happens in your browser
- 3Copy the result or download as a text file
What to do next
About XML Validator
XML Validator is part of a collection of single-purpose developer utility tools. Check XML well-formedness by validating tag matching, nesting, and angle bracket balance. Each tool is intentionally narrow — it does one thing well rather than offering many overlapping features — which makes the common path predictable and the result easy to verify before you download or copy it.
Architecturally, XML Validator is a single-page client. The processing layer is standard browser APIs; the UI is a thin React shell on top. Inputs flow through the engine and the output is returned to the browser as a Blob you can save or copy. The 0 MB cap is the only hard limit and it exists to keep memory usage stable on every device.
XML Validator performs the transformation entirely inside the JavaScript runtime. Your file lives in the tab's memory while the engine works on it; the result lives in the tab's memory until the browser triggers the download. Both are released when the tab closes, the way every browser tab releases its memory.
The heaviest users of XML Validator tend to be frontend developers prepping fixtures, engineers debugging API payloads and data analysts wrangling JSON. Each group brings slightly different expectations to the tool, but the same single-page architecture serves every one of them with the same response time.
Reach for XML Validator when you need a predictable result on a single file. The page works on the first visit, the controls are visible without a menu, and the output is delivered the moment the engine finishes.
The architecture imposes only the limits the browser itself imposes. The published 0 MB ceiling is conservative; most modern devices comfortably handle inputs up to that size, and the cap exists so the tool degrades gracefully on phones and budget laptops rather than running out of memory.
For multi-step jobs, XML Validator sits next to XML to CSV Converter, XML to YAML Converter, and XPath Tester. None of them depend on each other — you can use XML Validator on its own — but together they cover the common variations of the task this page exists to handle.
XML Validator is honest about scope: it handles a single, well-defined developer utility step. Specialist edge-case work — uncommon formats, very large inputs, or pipelines that need scripting — is what dedicated desktop apps are for. This page handles the common case quickly.
The output handed back by XML Validator is the output file. If you would prefer to keep the result in the browser instead of downloading it, you can copy it from the result panel and paste it directly into another tab — useful when the next tool in your workflow expects pasted text rather than a file.
Some background on the design choices behind XML Validator: every option you see on the page is there because a real workflow needs it, and every option that is not shown has been deliberately omitted to keep the common case fast. The bias is toward minimal-but-complete.
As a single-page tool, XML Validator stays focused on one developer utility step. Multi-step workflows are composed by chaining adjacent tools — each tool produces a standard file the next one can read directly, so a longer pipeline is just a sequence of short tab-and-tab visits.
If you want to get the most out of XML Validator, three small habits help. Drag-and-drop is faster than the file picker once you get used to it. The keyboard shortcut for downloading the result is whatever your browser uses for "save link as," because the result is a normal download. And if you are working on a sensitive file, processing in an Incognito or Private window is a good extra layer — it leaves no trace in browser history when the tab closes.
Common gotchas worth flagging: the supported formats are listed in the upload area. The 0 MB ceiling is per-file, not per-session; you can run as many separate jobs as you like, but a single oversized input will be rejected on load.
XML Validator is intentionally narrow in scope so the common case is fast and the result is predictable. If you ever need a variation it does not cover, browse the rest of the catalog — there is a good chance an adjacent tool already exists, and switching between tools is just a matter of opening another tab.
How it works
- 1Open the XML Validator workspace above. The interface is a single page, so there is nothing to navigate.
- 2Add your developer input by dropping it onto the page or browsing for it.
- 3Pick any non-default settings you need. Most users leave the defaults alone for the first run and only revisit if the result needs tuning.
- 4Trigger processing. standard browser APIs reads your input, applies the transformation, and writes the result back into the page.
- 5Download the result. The file is generated in your browser and saved through your normal download flow.
- 6Re-run with different settings as often as you want. Each run produces a fresh output and the original file on disk is never modified.
Common use cases
- Decode a token to confirm its claims during a debugging session using XML Validator.
- Inspect a regex against a test string before committing it.
- Hash a string for a quick reproducibility check.
- Inspect a payload during local development without writing a script.
- Validate a config blob before pushing to staging.
- Generate a quick fixture without leaving the browser.
- Compare two API responses to spot a regression.
- Convert between data formats while wiring up an integration.
FAQ
What does well-formed mean?
Well-formed XML has properly nested and matched tags, balanced brackets, and correct syntax.
Does it validate against a schema?
No — this checks structural well-formedness only, not DTD or XSD schema validation.
Does it handle CDATA sections?
CDATA sections are passed through. Tag matching focuses on regular element tags.
What about namespaces?
Namespaced tags (e.g. ns:tag) are recognized and matched correctly.
Can I validate large files?
The tool works in-browser, so very large files may be slow. It handles typical documents well.
Is my data safe?
All processing happens in your browser.
Which file formats does XML Validator accept?
The accepted formats are listed in the upload area on the tool itself. If your input is in a format that is not directly supported, convert it first using one of Favtoo's converter tools — every Favtoo converter outputs a file that is a clean input to the next tool in the chain.
Is there a programmatic version of XML Validator?
XML Validator is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (standard browser APIs) is open-source and can be used directly from your own code.
What should I do if XML Validator fails on my file?
Failures usually fall into one of three buckets: the input is in an unsupported format, the input is over the size cap, or the input is structurally malformed (a truncated download, a partial export, or a stream the engine does not recognise). The first two are easy to confirm — check that your file is in a supported format and that it is below 0 MB. For the third, opening the file in its native viewer first is the fastest way to confirm the source is intact.
Is XML Validator licensed for business use?
XML Validator can be used for personal and commercial work alike — there is no separate "business" licence to purchase. The output you generate is yours to use however you want, including in client deliverables, internal documents, or commercial products. Favtoo's only ask is fair, individual use; the tool is not designed to be embedded as a backend service or wrapped behind an API for resale.
Can I use XML Validator offline?
Once the page is loaded, XML Validator can complete jobs without an active internet connection — the engine is bundled with the page, so there is no per-job network call. The initial page load does require a connection (to fetch the static assets), but after that you can disconnect entirely and the tool will still work. This is a side-effect of the local-first architecture, not a deliberate "offline mode" feature.
How accurate is XML Validator?
XML Validator is built on standard browser APIs, which is the same class of engine used by professional developer utility pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.
How is XML Validator different from desktop apps that do the same thing?
Desktop apps usually have more advanced features but require installation, maintenance and (often) a licence. Paid online tools are convenient but route your file through their servers and gate downloads behind accounts. XML Validator sits in between: free, instant, and private, but intentionally narrow in scope. For one-off jobs and the common developer utility operations, it is usually the lowest-friction choice; for highly specialised work, a dedicated app is still the right answer.
Are there any hidden fees with XML Validator?
XML Validator is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
Does XML Validator upload my file to a server?
Your file is processed inside your browser by standard browser APIs. The engine reads the file's bytes from your tab's memory, computes the result, and writes the result back into the tab. You can confirm what the page does by opening developer tools and watching the Network tab during a run — the requests you see are for the tool's static assets only.