HTML vs OPML
A detailed comparison of HTML Document and OPML Outline — file size, quality, compatibility, and which format to choose for your workflow.
HTML Document
Documents & TextHTML is the standard markup language for web pages. As a conversion target or source, it carries text content with structural and formatting information that can be extracted or repurposed.
About HTML filesOPML Outline
Documents & TextOPML (Outline Processor Markup Language) is an XML format for structured outlines and lists. It is most widely used for exchanging RSS feed subscription lists between podcast apps and feed readers, and for hierarchical note-taking.
About OPML filesStrengths Comparison
HTML Strengths
- Universal — every browser, OS, email client, and document reader displays HTML.
- Plain text, human-readable, grep-able, and diffable in git.
- Flexible — pages render even with broken or partial markup (error-tolerant parser).
- Carries structure, styling (CSS), and behavior (JavaScript) in one file.
- Accessibility-friendly when written with semantic tags and ARIA attributes.
OPML Strengths
- Standard RSS subscription interchange format.
- Simple XML — easy to parse and generate.
- Highly extensible via arbitrary attributes.
- Supported by every major outline and RSS tool.
Limitations
HTML Limitations
- Error tolerance allows sloppy markup to hide real bugs.
- Rendering depends on browser engine — pixel-perfect cross-browser output is an art form.
- Security-sensitive — unsafe HTML can execute scripts or leak data (XSS vulnerabilities).
- File size for equivalent structured data is larger than JSON or XML due to tag verbosity.
- No built-in typing or schema — contract between server and client is informal.
OPML Limitations
- XML verbosity — larger than a JSON-based equivalent.
- Specification is loose — different tools disagree on edge cases.
- Primary use (RSS reading) has shrunk dramatically since Google Reader.
- No strong central stewardship.
Technical Specifications
| Specification | HTML | OPML |
|---|---|---|
| MIME type | text/html | text/x-opml |
| Extensions | .html, .htm | — |
| Standard | HTML Living Standard (WHATWG) | OPML 2.0 (2006) |
| Character encoding | UTF-8 (recommended) | — |
| Element count | ~110 in current spec | — |
| Extension | — | .opml |
| Format | — | XML with nested <outline> elements |
| Primary use | — | RSS subscription interchange |
Typical File Sizes
HTML
- Hello-world page < 1 KB
- Blog post (rendered HTML) 5-40 KB
- Modern SPA (initial HTML shell) 50-200 KB
- Full archived web page (with inline assets) 500 KB - 10 MB
OPML
- Typical RSS reader export (50 feeds) 5-30 KB
- Deep outline (Scrivener novel plan) 20-200 KB
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Frequently Asked Questions
HTML (HyperText Markup Language) is the core language of the web, created by Tim Berners-Lee in 1993. An HTML file is plain text describing structure (headings, paragraphs, links, images), optionally with styling (CSS) and interactivity (JavaScript). Every web page you visit is rendered from HTML.
HTML files open in every web browser by double-clicking. To edit, use any text editor (Notepad, VS Code, Sublime Text) or a visual editor (Dreamweaver, Pinegrow). Mobile browsers also render HTML files from local storage.
Use KaijuConverter's HTML-to-PDF converter, or print the page from your browser and choose "Save as PDF". For pixel-perfect conversion with page breaks, dedicated tools like wkhtmltopdf or Puppeteer give more control.
Markdown for authoring — it's faster to write, version-control-friendly, and renders to HTML via static-site generators. HTML for delivery and complex layouts where you need full control over styling, forms, and interactivity. Most modern blogs write in Markdown and publish as HTML.
Browsers implement CSS and JavaScript slightly differently, especially for cutting-edge features. Use a CSS reset, test in Chrome/Firefox/Safari, and tools like caniuse.com to check browser support. Modern frameworks (Tailwind, Bootstrap) normalize most cross-browser quirks automatically.
HTML itself is safe, but embedded JavaScript can perform malicious actions (redirects, form hijacking, cryptomining). Only open HTML attachments from trusted sources. Modern browsers sandbox local HTML files to limit their access to your system.