Markdown Linter
Check Markdown for common style issues — headings, whitespace, tabs, and more — in your browser
Examples
Heading level jump
# Title
### Skipped a level
Some content here.
L2 MD001 Heading level jumps from h1 to h3MD001 catches heading increments that skip a level (H1 → H3). It only fires inside the body of the document, never on the first heading.
Multiple consecutive blank lines
# Title
Paragraph here.
Next paragraph after extra blanks.
L5 MD012 Multiple consecutive blank linesMD012 reports the second of any two adjacent blank lines, which is the line you can delete to collapse the gap to one.
Trailing whitespace and a hard tab together
# Heading
indented with a tab
L1 MD009 Trailing whitespace
L2 MD010 Hard tab found — use spacesMD009 and MD010 run independently, so a single line can produce both — trailing spaces plus a tab character. Content inside fenced code blocks is ignored, so a literal tab in a code sample is fine.
About this tool
Markdown is forgiving, which is exactly why inconsistencies creep in: headings that jump levels, hard tabs mixed with spaces, lines that wrap at odd widths, and stray trailing whitespace. markdownlint codifies the community conventions that keep documents clean and render predictably across tools.
This linter runs a focused subset of those rules — MD001 (heading increment), MD009 (trailing whitespace), MD010 (hard tabs), MD012 (multiple blank lines), MD013 (line length), MD018 (space after #) and MD026 (trailing punctuation in headings) — and reports each issue with its line number, the rule id, and a plain-English explanation. Code blocks are skipped, so nothing inside a fenced block is flagged.
Linting runs entirely in your browser. Your document is never uploaded, so it's safe to lint README files, documentation, or draft posts in place.
How to use
Paste your Markdown
Enter or paste the document into the input pane.
Review the findings
Each issue shows its line number, rule id, and a short explanation; code blocks are ignored.
Fix and re-check
Correct the flagged lines and watch the issue list update live.
Use cases
Pre-commit check on a documentation repo
Run the linter over a README or docs folder before opening a PR; copy the rule ids (MD001, MD009…) into a CI markdownlint config to enforce the same rules automatically.
Cleaning up a legacy Markdown corpus
Paste a long document and use the per-line findings to bulk-replace hard tabs with spaces and collapse triple blank lines — the rules match the same findings markdownlint itself reports.
Teaching Markdown style
Show trainees a deliberately messy snippet and the issue list that comes back; each rule id maps to a single rule on markdownlint's docs.
Validating a blog post draft
Paste the post, fix the flagged lines, and confirm the issue list is empty before publishing. Lint runs locally so drafts never leave the browser.
Checked rules
| Rule | What it catches |
|---|---|
| MD001 | Heading levels that skip (e.g. H1 → H3) |
| MD009 / MD010 | Trailing whitespace and hard tabs |
| MD012 | Multiple consecutive blank lines |
| MD013 | Lines longer than the configured width |
| MD018 / MD026 | Missing space after # and trailing heading punctuation |
Content inside fenced code blocks is skipped.
Common mistakes
Mistake:Treating warnings as fatal and rewriting readable prose.
Fix:MD013 (line length) and MD026 (trailing punctuation) are stylistic. Disable the rules you don't care about in your own pipeline rather than mass-editing the prose.
Mistake:Expecting HTML mistakes to be flagged.
Fix:This linter targets Markdown style only. For HTML inside Markdown, use an HTML validator or rely on the Markdown renderer's own diagnostics.
Mistake:Pasting a document and assuming the first heading is checked by MD001.
Fix:MD001 only fires on transitions, not on the document's first heading — there is no previous level to compare against. That matches the upstream markdownlint behaviour.
Mistake:Hard-coding four spaces in nested lists.
Fix:Lists are outside the linter's scope here. Configure your Markdown renderer and your editor (or a full markdownlint config) for the indentation depth you want.
Frequently asked questions
Related guides
Markdown to HTML: A Developer's Guide
Why Markdown became the default for documentation, the GitHub-Flavored extensions that go beyond CommonMark, and how to keep the rendered HTML safe from XSS when you embed it.
Writing Clean Markdown: Rules Worth Following
The small Markdown style rules that keep documents consistent, diff-friendly, and safe to render — and how to catch violations automatically.
References & standards
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