HTML Encoder / Decoder

HTML encoding converts characters that have special meaning in HTML into their corresponding HTML entity representations, so they are rendered as literal text in the browser rather than being interpreted as HTML markup. The characters that must be encoded include the less-than sign (which becomes <), the greater-than sign (which becomes >), the ampersand (which becomes &), and the double quote (which becomes ").

HTML encoding is a specific application of the broader concept of output escaping — the practice of transforming data to make it safe for a specific output context. The same principle applies to SQL queries (parameterised queries), JavaScript strings (JS escaping), URLs (URL encoding), and shell commands (shell escaping). Each output context has its own set of dangerous characters and its own escaping convention.

HTML encoding is essential whenever user-supplied or external content is displayed in a web page. If a user submits a form field containing a script tag and your application renders it without encoding, the script executes in every visitor browser — this is a Cross-Site Scripting (XSS) attack, one of the OWASP Top 10 most critical web security vulnerabilities. Properly encoding all dynamic content before inserting it into HTML is the primary defence against XSS.

Beyond security, HTML encoding is required when displaying source code in documentation, tutorials, and technical blogs. If you include a code snippet containing HTML tags in a web page, those tags must be encoded or the browser will render them as markup instead of displaying them as text. Technical platforms like Stack Overflow, GitHub, and MDN use HTML encoding to safely display millions of code snippets submitted by users.