HTML Validator — Tag Matching Check
Validate HTML for tag matching, nesting errors, unclosed tags, and missing alt attributes.
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 HTML Validator
HTML Validator performs html validator as a focused single-page utility. Validate HTML for tag matching, nesting errors, unclosed tags, and missing alt attributes. Defaults are tuned for the common case so the first run is one click, with every option that matters exposed for the moments you need to fine-tune the result.
The processing pipeline is straightforward: your input is parsed by standard browser APIs, transformed according to the options you select, and serialised back into a downloadable result. The 0 MB per-file ceiling matches what a typical browser tab can handle without paging to disk.
HTML Validator sees the most use from devops engineers crafting one-liners and site reliability engineers triaging logs, but the design is intentionally generic enough that you do not need a specialist background to get a good result. The defaults aim at the most common case so a first-time user can get the right output without changing any settings.
The execution model is straightforward: your file is bytes in the tab's memory, the engine reads those bytes, computes the result, and hands the result back to the browser. The transformation happens locally, which is why the tool keeps working when your network connection drops mid-job and why it produces the same result every run for the same input.
Reach for HTML 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.
Once the engine finishes, the output is offered as an immediate download. There is no preview gate, no email-wall, and no "register to download" intermediary — the file is yours the moment it is ready.
A practical note on limits: HTML Validator accepts inputs up to 0 MB per run, and the tool processes one input at a time to keep memory usage predictable. If you ever bump into the ceiling, the cause is the size of the input.
As a workflow component, HTML Validator is the part you reach for when a single, well-defined developer utility step needs to happen. It performs that step and returns a standard file you can carry into the next part of your pipeline.
Some notes on the design of HTML Validator. The page is intentionally narrow: one input, the controls relevant to the task, and one output. Adding unrelated features would make the common case slower for the majority of users, so the surface is held to what people actually use.
From a product perspective, HTML Validator is one of the simplest possible expressions of "do one thing well." The catalog contains dozens of related tools that each handle a slightly different developer utility task, and every one is a separate page rather than a tab inside a larger app. That separation keeps each tool fast to load and easy to bookmark.
HTML Validator is built around the moment of need: a focused page you open when you have a specific task, complete the task, and close. The catalog contains many adjacent tools so the same model serves the surrounding parts of a typical developer utility workflow.
Useful patterns when working with HTML Validator: keep the input file open in another tab so you can compare against the result; give the output file a descriptive name when saving so you can find it later (the default name is sensible but generic); and treat each run as independent — the tool has no concept of "history", which means you cannot accidentally pollute one job with leftovers from another.
When something goes wrong, the cause is usually one of three things: a malformed input, a browser that is out of memory, or a corporate proxy that is interfering with the page's static assets. The first two are easy to diagnose; the third typically requires asking your IT team to allow standard browser APIs to load.
If HTML Validator solved your problem, sharing the page link with someone who has the same problem is the most useful thing you can do. The catalog grows mostly through word of mouth; visitors arriving through a recommendation tend to be the ones the tool serves best.
How it works
- 1Open HTML Validator in your browser. The page loads quickly and the tool is ready to use the moment it becomes interactive.
- 2Drop a developer file onto the upload area, or click to pick one from your device.
- 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.
- 6Run additional jobs as needed. The same controls and defaults apply on every run.
Common use cases
- Pretty-print a minified blob during incident triage using HTML Validator.
- Generate a quick fixture without leaving the browser.
- Convert between data formats while wiring up an integration.
- Encode binary content for transport in a JSON body.
- Generate boilerplate from a single specification line.
- Compare two API responses to spot a regression.
- Hash a string for a quick reproducibility check.
- Validate a config blob before pushing to staging.
- Inspect a payload during local development without writing a script.
FAQ
What does it check?
Tag matching and nesting (opening/closing pairs), void element recognition, and img alt attributes.
Does it check for deprecated elements?
No — it focuses on structural validity, not HTML5 deprecation warnings.
Does it handle self-closing tags?
Yes — void elements (br, img, input, etc.) and self-closing syntax (/>) are handled correctly.
Will it check CSS or JS?
No — only HTML structure is validated. Use the CSS validator for stylesheets.
Does it enforce doctype?
No — it works with HTML fragments and does not require a <!DOCTYPE> declaration.
Is my data safe?
All processing happens in your browser.
Does HTML Validator work on a phone or tablet?
HTML Validator runs in any modern mobile browser — Safari, Chrome, Firefox and the in-app browsers in most messaging apps all support the underlying APIs. Performance depends on the device: a recent phone handles typical inputs nearly as fast as a laptop, while older devices may take a few seconds longer near the 0 MB ceiling. The interface lays out cleanly on small screens, so you do not need to pinch-zoom to see the controls.
Do I need to install anything to use HTML Validator?
No installation is needed. HTML Validator runs as a normal web page, with no browser extension, no native helper, and no separate desktop client to download. That is partly a privacy choice — extensions can request broad permissions, while a regular page is sandboxed by default — and partly a convenience one: you can use HTML Validator on any computer you have temporary access to without leaving anything installed on it.
Does Favtoo keep a copy of files I process with HTML Validator?
Favtoo keeps no copy of your file because Favtoo never receives your file. HTML Validator runs entirely in your browser, the input is held only in your tab's memory, and closing the tab discards it. There is no opt-in cloud history, no "recent jobs" panel synced to an account, and no server-side retention to configure — the architecture simply has nowhere for your file to be stored.
Are there any restrictions on using HTML Validator at work?
HTML 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.
Why did HTML Validator reject my input?
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.
Which file formats does HTML 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.
What does HTML Validator do that command-line tools do not?
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. HTML 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.
Can I call HTML Validator from a script?
HTML 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.