Overview
Introduction
Unlike JSON or YAML, Markdown doesn't have a syntax a parser can flatly reject, an unclosed bracket or a stray asterisk just renders as literal text. That makes structural mistakes easy to miss until the document actually renders somewhere.
This validator checks for the specific patterns that do cause visible problems: a code fence that swallows the rest of the document, headings that skip a level, and accessibility gaps like images with no alt text.
What Is Markdown Validator?
A focused Markdown linter covering four checks: unclosed fenced code blocks, heading levels that jump by more than one, multiple top-level (#) headings, and images with empty alt text.
These mirror well-established rules from markdownlint (MD001, MD025, MD045), not invented heuristics, so the feedback matches what other Markdown tooling would also flag.
How Markdown Validator Works
The document is scanned line by line, tracking whether the current line is inside a fenced code block so headings and images inside example code aren't mistakenly flagged.
Each issue is reported with its line number and a severity of error (breaks rendering) or warning (valid but worth fixing), rather than a flat pass/fail.
When To Use Markdown Validator
Use it before committing a README or doc to catch an unclosed code fence, the single most common Markdown mistake that visibly breaks a page.
It's also useful as an accessibility pass, catching images that are missing alt text before publishing.
Often used alongside Markdown Formatter & Pretty Printer, Markdown Header Extractor and Markdown to HTML Converter.
Features
Advantages
- Catches the specific mistake (an unclosed fence) that silently breaks the rest of a rendered document.
- Flags heading structure and accessibility issues most other online Markdown tools skip.
- Runs entirely client-side with no upload required.
Limitations
- Doesn't check whether linked or embedded URLs actually resolve, since that would require a network request.
- Doesn't cover the full markdownlint rule set (line length, consistent emphasis style, etc.), only the four checks listed above.
Examples
Best Practices & Notes
Best Practices
- Run this on any Markdown before committing it, since an unclosed fence is easy to introduce and easy to miss visually in an editor.
- Treat warnings as a checklist for a documentation review, not a hard blocker.
- Pair with the Markdown Formatter to fix spacing issues alongside structural ones.
Developer Notes
Fence tracking uses the same opening-marker-must-match-closing-marker logic as the Markdown Formatter, so a ~~~ fence containing a literal ``` line isn't mistakenly closed early. Heading level tracking resets per document, comparing each heading's depth only against the immediately preceding heading, not the whole document's structure.
Markdown Validator Use Cases
- Catching an unclosed code fence before publishing a README
- Auditing images for missing alt text before a documentation release
- Checking heading structure follows a logical, non-skipping hierarchy
Common Mistakes
- Assuming a clean result means the Markdown is fully correct; this checks structure, not prose quality or broken links.
- Not noticing a fence was left open until the rest of the document renders as a single code block somewhere else.
Tips
- If you see an unclosed-fence error, count your ``` markers, an odd number anywhere in the document means one is missing.
- Fix errors first; they're the ones that visibly break rendering, unlike warnings.