Markdown Validator
Validate markdown syntax and detect common issues
Input Markdown
Validation Report
What is a Markdown Validator?
A markdown validator is a tool that checks whether markdown files conform to syntax specifications and renders without errors. Unlike a linter which enforces style preferences, a validator focuses on correctness—ensuring your markdown is structurally sound and will display properly on any platform. The validator identifies broken elements like unclosed code fences, malformed links, invalid heading levels, and table alignment problems that would otherwise cause rendering failures or display issues.
The CommonMark specification provides the reference standard that all markdown validators use. CommonMark is a rationalized and well-documented version of markdown that eliminates ambiguities from the original format, making it easier to build reliable parsers and validators. This browser-based validator checks your content against CommonMark rules instantly, catching problems before you publish to platforms like GitHub, GitLab, or static site generators.
When you validate markdown, you're essentially asking: "Will this render correctly everywhere?" A validator catches subtle issues that invisible until preview time—unclosed code fence blocks that expose raw markdown, link syntax errors, broken table structures, and image paths that fail to load. This is particularly important in documentation workflows where a single rendering error can confuse readers or break navigation.
How to Use the Markdown Validator
Follow these steps to validate your Markdown. Each step uses the actual buttons and panels on this page.
Paste, Upload, or Load Sample
Paste Markdown into the left Input Markdown panel, or click Upload to load a .md file. Click Sample to see example subscriber API docs with intentional issues for validation. Checks against CommonMark.
View Validation Report
The right Validation Report panel lists errors and warnings with line numbers. Catches unclosed code fences, malformed links, invalid headings, tables.
Copy or Download Report
Click Copy or Download to use the report. Use Clear to start fresh. All processing runs in your browser.
When You'd Actually Use This
Pre-Publication Verification
Before publishing content to GitHub, GitLab, Notion, or a static site generator, run validation to catch rendering errors. Catching broken code fences or malformed tables before publication prevents embarrassing errors in production documentation.
API Documentation Quality
Validate markdown used in API docs, OpenAPI specifications, and SDK documentation. Ensures code examples in fenced blocks render properly, links work correctly, and tables displaying parameters are structurally sound. Tools like Swagger rely on valid markdown.
Blog and Content Migration
When migrating blog content from HTML or other formats to markdown, validate that the converted markdown is structurally correct. Catches conversion artifacts like escaped characters, broken links, and malformed tables.
Accessibility and Semantic Compliance
Ensures your markdown follows semantic structure with proper heading levels and alt text on images. Valid markdown structure is crucial for screen readers and produces more accessible final output when rendered to HTML.
Common Questions (FAQ)
What markdown dialect does this validator support?
This validator strictly follows the CommonMark specification, which is the most portable and standardized markdown format. GitHub Flavored Markdown (GFM) extends CommonMark with tables, strikethrough, and task lists. For GFM-specific features, validate against GitHub's extensions.
What's the difference between validation errors and warnings?
Errors are structural problems that prevent correct rendering—unclosed code fences, broken link syntax, invalid heading sequences. Warnings flag issues that might not break rendering but indicate poor practices or potential accessibility problems. Both should be addressed before publishing.
Can I validate markdown for GitHub specifically?
This tool validates CommonMark compliance. GitHub uses GitHub Flavored Markdown, which extends CommonMark with tables, task lists, and strikethrough. Content that passes this validator will work on GitHub, but GitHub-specific features won't be validated here. Check GitHub's markdown documentation for flavor-specific rules.
Does the validator check spelling or grammar?
No, the validator focuses only on markdown syntax and structure. For spell-checking and grammar, use separate tools like Grammarly or IDE extensions. The validator ensures your markdown is structurally valid and will render correctly.
How do I fix validation errors?
Each error includes the line number and description. Common fixes: close unclosed code fences with matching backticks, fix link syntax by ensuring brackets and parentheses match, align table columns properly, add missing alt text to images, and ensure headings follow logical sequence (H1 → H2 → H3, not H1 → H3).
Is validation data sent to any server?
No. All validation runs entirely in your browser using JavaScript. Your markdown content is never transmitted anywhere, making this safe for proprietary, private, or sensitive documentation.