ToolGrid — Product & Engineering
Leads product strategy, technical architecture, and implementation of the core platform that powers ToolGrid calculators.
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Validate Markdown syntax for errors, check link references, verify image paths, detect malformed tables, validate code block formatting, and ensure Markdown follows standard specifications (CommonMark, GitHub Flavored Markdown) for proper rendering.
Note: AI can make mistakes, so please double-check it.
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Common questions about this tool
Paste your Markdown content into the validator, and it automatically checks for syntax errors, malformed links, broken image references, invalid table structures, and formatting issues. The tool provides detailed error messages to help you fix problems.
The validator checks Markdown against CommonMark and GitHub Flavored Markdown (GFM) specifications. It validates standard Markdown syntax, extended features like tables and task lists, and ensures your Markdown will render correctly in most Markdown processors.
The validator detects syntax errors (malformed headings, broken links), invalid table structures, unclosed code blocks, broken image references, invalid link syntax, and formatting issues that would prevent proper Markdown rendering.
Yes, the validator checks link syntax (both inline and reference-style links), verifies image path formatting, validates link reference definitions, and identifies broken or malformed links that would cause rendering issues.
Validating Markdown syntax helps ensure proper rendering, but different Markdown processors may have slight variations. The validator checks against standard specifications, so validated Markdown should render correctly in most Markdown viewers and processors.
Paste or type your markdown into the editor and the tool continuously runs it through `validateMarkdown`, which scans each line for common issues like missing spaces after `#` headers, malformed list markers, empty image alt text, and multiple blank lines. Detected problems are surfaced as `MarkdownIssue` objects with severity, rule codes, and line numbers, so you can quickly see exactly what needs fixing.
As you edit, a debounced validation pass updates the issues panel; if no `MarkdownIssue` entries are found and your content is non‑empty, the interface shows a “Markdown is valid” success state. This means the text passes the current rule set implemented in `validateMarkdown`, covering basic syntax and style checks for headings, lists, images, and spacing, though it does not perform link checking or external reference validation.
For many issues the validator provides a `fixAction`, such as inserting a space after a `#` in headers or after list markers, or removing extra blank lines, which you can apply per issue using the Fix button. You can also click “Apply All Fixes” to run `applyAllSafeFixes`, which repeatedly re-validates and applies simple single‑line corrections in a controlled loop so the document is cleaned up without large, unpredictable edits.
Switch to Preview or Split mode in the toolbar to see your content rendered side by side with the issues list; the app passes the markdown through `marked.parse` and sanitizes it with `DOMPurify` before displaying it. This lets you confirm that fixes suggested by the validator still produce the visual layout you expect, while keeping rendering safe with an allowlist of tags and attributes.
If you click the Refactor button with non‑empty content, the tool sends your markdown to a backend `markdown-validator` AI service via `performAIRefactor`, which returns a refined version of the text. The editor replaces your content with that result while preserving validation behavior, but AI changes are applied only when you explicitly trigger them. Validator input limits are separate from AI: free plans allow up to about 200KB of markdown in the editor (or smaller files read locally), while paid plans allow much larger documents; AI Refine has its own character cap independent of those limits.
Verified content & sources
This tool's content and its supporting explanations have been created and reviewed by subject-matter experts. Calculations and logic are based on established research sources.
Scope: interactive tool, explanatory content, and related articles.
ToolGrid — Product & Engineering
Leads product strategy, technical architecture, and implementation of the core platform that powers ToolGrid calculators.
ToolGrid — Research & Content
Conducts research, designs calculation methodologies, and produces explanatory content to ensure accurate, practical, and trustworthy tool outputs.
Based on 1 research source:
Learn what this tool does, when to use it, and how it fits into your workflow.
This Markdown validator is an online Markdown validator and linter that checks your markdown text for syntax errors and style problems directly in your browser. You paste markdown content, select a flavor (CommonMark, GitHub, or GitLab), and the tool validates markdown syntax online, finding missing spaces after headings, list markers without spaces, images without alt text, and other issues, with each problem showing the line number and a message explaining what is wrong.
When you write markdown, small mistakes can break how it renders or make documentation harder to maintain. A heading without a space after the hash, a list marker without a space, an image without alt text, unbalanced headings, or extra blank lines can all cause problems. Finding these mistakes by hand is slow and easy to miss, so many people look for a way to validate Markdown syntax online free or check markdown for errors and broken links before committing README files.
This tool takes your markdown content, runs checks based on the flavor you choose, and lists all issues it finds in one place. You can use it as a Markdown linter to validate heading structure, list formatting, and basic link and image syntax, fix issues one by one or apply all fixes at once, preview how your markdown will look when rendered, and optionally ask for AI refactoring to improve your markdown. The tool updates results automatically as you type so you always see the current validation status.
The tool is for developers, writers, and anyone who writes markdown and wants a browser based Markdown validation tool without installing plugins or CLI linters. You only need to paste markdown and read the results; no extra setup is required, and it works well for README files, documentation, blog posts, and other technical content where clean, valid markdown matters.
Markdown validation means checking if your markdown text follows the rules of the format. Markdown uses simple syntax like hashes for headings, dashes for lists, and brackets for links. Each syntax element has rules. Headings need a space after the hash. List markers need a space after them. Images should have alt text for accessibility.
Different markdown flavors use slightly different rules. CommonMark is the standard specification. GitHub Flavored Markdown (GFM) adds features like tables and task lists. GitLab adds its own features. The validator adapts its checks based on which flavor you choose.
Common mistakes include missing spaces after headings or list markers, images without alt text, and multiple blank lines in a row. These mistakes may not break rendering but can cause inconsistent output or accessibility issues. A related operation involves formatting Markdown as part of a similar workflow.
Validation tools parse markdown and look for these problems. They do not render the final output; they only check syntax and style. They report issues with line numbers so you can find and fix them quickly.
Some problems are errors that will affect rendering. Others are warnings that might cause issues or are not best practice. Style issues are suggestions for consistency. Errors should be fixed. Warnings and style issues are suggestions you can address when you have time.
Before publishing: paste your markdown into the validator and check for errors. Fix errors before publishing so your content renders correctly. Use the preview to see how it will look.
Learning markdown: when you are learning markdown, use the validator to check your work. Read the messages to understand what went wrong and how to fix it. The fix actions help you learn correct syntax.
Code review: before sharing markdown with others, validate it to catch obvious mistakes. Fix errors so reviewers can focus on content instead of syntax.
Accessibility: the tool checks for images without alt text. Adding alt text makes your markdown accessible to screen readers. Fix these warnings to improve accessibility. For adjacent tasks, validating JSON syntax addresses a complementary step.
Consistency: the tool finds style issues like multiple blank lines. Fixing these makes your markdown consistent and easier to read.
Refactoring: when you want to improve your markdown, use the AI refactor button. It suggests better formatting and structure. Review the changes and keep what you like.
Preview: use the preview mode to see how your markdown will render. This helps you catch formatting issues before publishing.
The tool does not execute or run your markdown. It only checks syntax and style.
Line-by-line checking: the tool splits your markdown into lines and checks each line for problems. It uses regular expressions to find patterns like headings without spaces or list markers without spaces.
Heading check: the tool looks for lines that start with one to six hashes followed immediately by text without a space. When it finds this pattern, it reports an error and can add the missing space. When working with related formats, validating XML structure can be a useful part of the process.
List check: the tool looks for lines that start with dashes, asterisks, or plus signs (with optional indentation) followed immediately by text without a space. When it finds this pattern, it reports an error and can add the missing space.
Image check: the tool looks for image syntax with empty alt text brackets. When it finds an image without alt text, it reports a warning and can add placeholder alt text.
Blank line check: the tool looks for consecutive blank lines. When it finds two or more blank lines in a row, it reports a style issue and can remove the extra blank line.
Flavor selection: the tool uses the selected flavor to determine which checks to run. GitHub and GitLab flavors may have additional checks for extended features like alerts or tables.
Fix application: when you click Fix on an issue, the tool applies the fix function for that issue. The fix function modifies the markdown text and returns the updated version. The editor updates with the fixed text.
Apply all fixes: when you click Apply All Fixes, the tool finds all issues with fix actions, applies them one by one, and revalidates after each fix. It stops after 20 iterations or when no more fixable issues remain. This prevents infinite loops. In some workflows, validating YAML syntax is a relevant follow-up operation.
Preview rendering: the tool uses a markdown parser to convert your text to HTML. It sanitizes the HTML to prevent security issues. The preview shows the rendered HTML so you can see how your markdown will look.
AI refactoring: when you click Refactor, the tool sends your markdown to a backend service. The service analyzes it and returns improved markdown. The tool replaces your content with the refactored version. If the service fails, it returns your original content.
Debouncing: the tool waits about 300 milliseconds after you stop typing before checking. This prevents checking on every keystroke and keeps the tool responsive.
| Limit | Value | Reason |
|---|---|---|
| Maximum input size | 500 KB | Keeps validation fast and prevents browser slowdowns. |
| Maximum AI refactor size | 100 KB | Limits content sent to AI service for processing. |
| Supported flavors | 3 | CommonMark, GitHub (GFM), GitLab. |
| Validation delay | 300 milliseconds | Waits after typing stops before checking to avoid checking too often. |
Select the correct flavor before pasting markdown. Different flavors have different rules. GitHub supports tables and task lists. CommonMark is more strict. Selecting the wrong flavor may cause false errors or miss real problems.
Fix errors first, then warnings. Errors affect rendering. Warnings may not break it but can cause issues. Style issues are suggestions you can address later.
Use line numbers to find issues quickly. The editor shows line numbers, and issues list the line where each problem was found. For related processing needs, validating HTML code handles a complementary task.
Use fix actions to learn correct syntax. When you click Fix, watch how the markdown changes. This helps you learn the correct syntax for next time.
Preview your markdown before publishing. Use the preview mode to see how your markdown will render. This helps you catch formatting issues early.
Use AI refactoring for improvements. When you want better formatting or structure, use the Refactor button. Review the changes and keep what you like. The refactored version replaces your content, so copy it first if you want to compare.
Keep markdown within size limits. Very large files may be slow to validate or may exceed the size limit. If your markdown is too large, split it into smaller sections or remove unnecessary parts.
The tool checks syntax, not content. It finds missing spaces and style issues, but it does not check if your content is correct or makes sense. After fixing syntax errors, review your content for accuracy.
Some checks are basic. The tool checks for common problems but does not verify all markdown rules. Complex markdown may have issues the tool does not catch. Always preview your markdown to verify it renders correctly.
Fix actions may not always be perfect. The tool applies fixes automatically, but you should review them. Some fixes may change your markdown in ways you do not want. Use fix actions as a starting point, then edit as needed.
We’ll add articles and guides here soon. Check back for tips and best practices.
Summary: Validate Markdown syntax for errors, check link references, verify image paths, detect malformed tables, validate code block formatting, and ensure Markdown follows standard specifications (CommonMark, GitHub Flavored Markdown) for proper rendering.