Batch processing flow for choosing the conversion direction
The Markdown ⇔ HTML Bidirectional Converter is a tool that allows you to switch between Markdown to HTML and HTML to Markdown directions.
Rather than converting in real-time with input, you select the direction, enter your text without a strict character limit, and then press the conversion button.
The result is displayed on the right, where you can copy, load a sample, or use the HTML preview. Note that changing the direction clears both input and output; there is no swap function to move the result to the next input.
"Bidirectional conversion" means you can choose between the two directions, not that any document can be round-tripped without information loss.
Markdown and HTML can represent different amounts of information, and the elements handled by this tool are limited to basic document structures.
Even if you revert the same content from Markdown to HTML and back to Markdown, spaces, line breaks, symbols, and numbers may not match the original string.
Changing the conversion direction simply clears both the input and output fields. It does not transfer the result to the input area, and there is no function to save the original input as a history.
Be sure to save necessary manuscripts elsewhere before changing directions.
Supported range for converting Markdown to HTML
Markdown to HTML converts block and inline elements using a custom, lightweight string process. Supported blocks include H1 to H6 headings, paragraphs, single-level bulleted lists, ordered Markdown lists that preserve their initial number via the ol start attribute, and code blocks enclosed in three backticks.
Blockquotes are properly recognized from escaped > prefixes at the beginning of each line. For inline elements, it handles code, links, images, bold text, and italics.
Link schemes are restricted to http, https, and mailto (with images restricted to http and https only), control characters and protocol-relative double slashes are rejected, and attribute quotes are escaped without certifying destination trust.
Horizontal rules, tables, raw HTML, and full CommonMark remain unsupported.
To prevent markup collisions, the Markdown forward parser shelters fenced code blocks and inline code before applying later replacement rules, restoring the preserved code at the end while escaping literal &, <, and > characters.
This ensures that subsequent regular expressions for emphasis, links, or other elements do not alter the contents inside code sections.
In addition, blockquote detection accurately recognizes escaped > characters at the start of a line to generate blockquote elements.
Input such as script tags is converted into plain escaped text rather than executable markup, though raw HTML, tables, and full CommonMark are not supported.
Link and image URLs are validated during Markdown parsing so that explicit schemes are rejected unless they use http:, https:, or mailto: for links, or http: and https: for image sources.
The parser also filters out control characters and protocol-relative URLs beginning with //, while escaping quotes and attribute characters to prevent attribute breakout.
However, these syntactic checks do not certify the trustworthiness of the destination website, nor do they guarantee the safety of linked external image assets or target content.
Elements reconstructed when converting HTML to Markdown
For HTML to Markdown, it parses the input HTML into a DOM tree using the browser's DOMParser and recursively examines each node. It replaces headings, paragraphs, line breaks, strong/b, em/i, code/pre, blockquote, a, img, and ul/ol/li with the corresponding Markdown syntax.
Horizontal rules are unsupported. Text nodes use their content, and general elements without a direct equivalent traverse their child elements' text.
For lists, list items (li) in an ordered list (ol) are always converted using a hyphen -, just like unordered lists. It does not preserve the start, reversed, or value attributes, nor does it keep the numbers.
It does not reproduce CSS counters, the type attribute, or the appearance of complex nesting. Visually check the item order and formatting after conversion.
For links and images, the URLs are extracted without validation, meaning it will create Markdown links even for unpermitted schemes.
Elements like script, style, template, noscript, iframe, object, and embed are not completely excluded, meaning their text content will remain in the output, although they are not executed as code.
This tool does not comprehensively diagnose threats in arbitrary HTML.
Attributes, CSS, and whitespace lost in round-trip conversion
HTML class, id, data attributes, event attributes, inline style, external CSS, and comments are not preserved in Markdown.
Structures without dedicated Markdown support, such as tables, forms, videos, audio, details, and SVG, cannot be reproduced in the same form as the original HTML.
Since DOMParser may correct incomplete HTML using browser rules, how tags are closed or the parent-child relationships of elements may change from the input string.
The Markdown side is also not fully compliant with CommonMark or GitHub Flavored Markdown. Extended syntaxes such as tables, task lists, footnotes, strikethrough, complex nesting, and autolinks are unsupported.
For list items containing multiple paragraphs or emphasis expressions with consecutive symbols, the results may differ from other Markdown parsers.
For this reason, this tool is best used as a draft for transferring a document's main headings, paragraphs, emphasis, links, lists, and code.
If you need to migrate designed HTML, convert an entire CMS, or require a lossless round-trip, please compare it with a dedicated converter that supports your target specifications.
Considering preview and sanitization separately
The HTML preview is an auxiliary function to check the basic appearance of the conversion result within the screen. The preview area uses a sandboxed iframe with custom CSS, so while input styles may remain, there is no guarantee that the host site's CSS will automatically apply.
Therefore, when the output code is pasted into another website, the appearance, link behavior, and image loading will not necessarily be the same.
The restrictions on scripts and external images applied to the preview frame do not carry over to the copied string.
Although Markdown link schemes and attribute quotes are validated during conversion, the reverse HTML-to-Markdown feature discards HTML tag markup while retaining the inner text found within <script> and <style> elements without executing it.
The utility does not act as an HTML sanitizer like DOMPurify, nor does it guarantee full CommonMark support. You must independently verify all allowed tags, attributes, Content Security Policy rules, destination targets, and accessibility requirements.
Never use this converter in publishing pipelines where untrusted third-party input is rendered directly without dedicated sanitization.
Seeing normal rendering in the preview does not prove compliance with HTML specifications or safety upon publication, as the SafeHtmlPreview limits do not guarantee the safety of the copied code.
Use developer tools or the inspection functions of the destination on the actual publication page to check for unintended tags, links, or layout breakdowns.
Conversion steps for easy checking without length limits
There is no strict 200,000 character limit for inputs, so it will accept very large texts. Because the conversion is executed all at once in the browser's main thread, the processing time will vary depending on the HTML depth, document volume, and device performance.
If you divide large documents into chapters or sections and process them after checking the supported elements in short ranges, it becomes easier to spot missing parts.
When starting from Markdown, first check the supported syntax with a sample, then paste and convert your manuscript. Compare the HTML code with the preview, and manually supplement unsupported parts like tables and nesting.
When starting from HTML, check whether heading hierarchies, links, and image alt text remain in the converted Markdown, keeping in mind that list numbers will be lost.
When converting documents containing code, pay attention to the opening and closing markers of fences and the boundaries of inline backticks.
Fenced blocks and inline code spans are protected before subsequent emphasis, link, and formatting replacements occur, and their contents are restored at the end with literal ampersands (&) and angle brackets (< and >) safely escaped as HTML entities.
Even so, the parser will not automatically repair forgotten closing delimiters or unsupported fence combinations, and raw HTML or tables remain unsupported.
You should verify sample sections to confirm that boundaries and code markup render as expected before processing an entire document.
Ordered lists converted from HTML will lose their numbers, start, reversed, and value attributes, turning entirely into hyphens.
For images, alt and src are migrated, and for links, the display text and href are migrated, but none of these URLs are validated.
If you need to retain surrounding attributes like title, class, or style separately, please keep the original HTML and supplement them manually.
When a difference is found in the conversion result, decide which one will be the original instead of repeatedly forcing round-trips.
By treating HTML as a derivative for distribution in an operation where Markdown is the original, or treating Markdown as a derivative for text extraction where HTML is the original, you can easily avoid accidents where lost information is overwritten with each conversion.
The converter awaits clipboard write completion before reporting success, provides a dedicated manual copy field on failure, and clears prior results upon text edits until converted again.
Because there is no auto-save or file history, please store the input manuscript and the conversion result separately. It is important to convert and display the final deliverable at the publication destination once more, rather than judging publication readiness solely on the results of this tool.