html-compiler.landing Open editor

html/compiler

HTML Compiler for Validating and Previewing HTML Code Online

There is no such thing as compiling HTML — the browser parses it directly — so this is where you watch the things around it compile: Pug into markup, SCSS into CSS, TypeScript into JavaScript.

Open the compiler Which compiler?
htmlparsed, not compiled
pug→ HTML
scss→ CSS, Dart Sass
typescript→ JavaScript
page.pugstyles.scss
doctype html
html(lang='en')
  body
    main.card
      h1= title
      each item in items
        li= item.name
compiled in 7ms

01 platform

What people mean by an HTML compiler

The phrase is a misnomer that the whole industry uses anyway, and being clear about it is more useful than playing along. HTML is a markup language: the browser parses it and builds a DOM. There is nothing to compile.

  • Parsed, not compiled — and that is the point

    HTML has no build step by design. A browser reads the tags and constructs a tree. That is why a .html file opens by double-clicking it and a .jsx file does not.

  • But the things around it do compile

    Pug compiles to HTML. SCSS, Sass and Less compile to CSS. TypeScript compiles to JavaScript. All three run in this tab, which is what a search for an HTML compiler is usually reaching for.

  • Validation is the other half

    What people also mean is: tell me whether my markup is correct. Unclosed tags, misnested elements, duplicate ids and missing required attributes are all catchable, and the editor catches them as you type.

  • Inspection is the third

    And sometimes it means: show me what the browser actually built. The rendered DOM differs from the source you typed — the parser inserts <tbody>, closes tags you left open — and seeing that difference explains a great many layout bugs.

02 pipeline

What actually gets compiled, and into what

Three real compilers run in this tab, none of them on HTML itself. Knowing which handles what is what makes an error message legible.

  1. Pug → HTML

    Indentation-based markup becomes tags. Mixins, loops and interpolation are resolved here, so a template error surfaces against the .pug file rather than as malformed output.

  2. SCSS → CSS

    Dart Sass compiles nesting, variables, mixins and @use in the browser. An undefined variable stops here with a line number rather than silently producing an empty rule.

  3. TypeScript → JavaScript

    Annotations are stripped so the browser can execute the result. This is a transform rather than a type check — types help you while editing and do not gate the preview.

  4. HTML → DOM

    The browser parses. No compiler is involved, which is why this stage cannot fail in the way the three above can: a malformed document still produces a tree, just not the one you meant.

03 deep dive

Catching markup problems before the browser hides them

The parser's error recovery is the reason bad HTML is so hard to debug. It never refuses — it guesses, silently, and the guess is usually not what you wanted.

Unclosed tags change the tree, not the render. A missing </div> nests everything after it one level deeper. The page still displays; your CSS selectors just stop matching.

Misnested elements get relocated. A <div> inside a <p> is illegal, so the parser closes the paragraph first. Your layout inherits a structure you never wrote.

Duplicate ids break more than styling. getElementById returns the first match, label-for associations point at the wrong control, and anchors jump to the wrong place.

Missing attributes fail quietly. An <img> without alt, an input without a label, a table without headers — none of these stop rendering, and all of them are real defects.

unclosed
tree changes silently
misnesting
parser relocates
duplicate ids
breaks lookups
a11y
checked inline
broken.html
<p>
  <div class="note">Illegal here</div>
</p>

<!-- the parser produces: -->
<!-- <p></p><div class="note">…</div><p></p> -->

04 features

What this compiles and checks

The surface below runs client-side. Anything that needs a Node build step is out of scope, and the FAQ says which.

  • Pug templates

    Full Pug: mixins, includes, extends, iteration and interpolation, compiled to HTML in the browser.

  • SCSS, Sass and Less

    Dart Sass compiled to WebAssembly handles nesting, variables, mixins, @use and @forward with no watcher.

  • TypeScript

    Annotations stripped so the browser executes plain JavaScript. Type-only imports and generics are handled.

  • Markdown

    Compiled to HTML for documentation pages, so a README renders beside the page it documents.

  • Live markup diagnostics

    Unclosed tags, misnesting, duplicate ids and missing required attributes flagged in the editor as you type.

  • The rendered DOM

    Inspect what the parser actually built against what you wrote — usually the fastest explanation for a layout that will not behave.

05 workflow

Which stage failed, and on which line

Three compilers, each failing on its own terms. This is what the pipeline says on a clean pass and on a broken one.

session
  • save → page.pug
  • pug → html ok 1.2 KB
  • scss → css fail Undefined variable $accent
  • styles.scss:14:11
  • define $accent → save
  • compiled in 7ms · preview repainted

  • Each compiler reports separately

    A Sass failure is not a Pug failure. Naming the stage narrows the search from the project to one file.

  • Positions point at your source

    Line and column refer to the file you wrote, never to the generated CSS or HTML.

  • A failed compile keeps the last good render

    The previous output stays on screen, so you keep the working version to compare against while you fix the broken one.

06 ecosystem

A page that mixes languages

A plain .html file can sit beside a React component and a Python script; XCODX picks the right handling per file rather than per project.

  • React
  • Vue
  • Angular
  • Svelte
  • SolidJS
  • Lit
  • HTMX
  • Vite
  • TypeScript
  • JavaScript
  • HTML
  • CSS
  • SCSS
  • Less
  • Pug
  • Tailwind
  • Bootstrap
  • Vuetify
  • Ant Design
  • shadcn/ui
  • Phaser
  • Node.js
  • Python
  • Java
  • C++
  • C#
  • Go
  • Rust
  • Ruby
  • PHP
  • Kotlin
  • Swift
  • SQL
  • Bash

07 languages

Every language the editor knows

Highlighting and IntelliSense cover 70+ languages. These are the ones XCODX also compiles or executes for you.

Compiled in your browser11

Zero build step. Transform runs on a Web Worker, so typing never blocks.

  • JSX
  • TSX
  • TypeScript
  • Vue SFC
  • Svelte
  • Angular (JIT)
  • SolidJS
  • SCSS
  • Sass
  • Less
  • Pug

Rendered natively8

Straight to the live preview, no transform in the way.

  • HTML
  • CSS
  • JavaScript
  • JSON
  • Markdown
  • Jupyter (.ipynb)
  • SVG
  • YAML

Executed on demand22

70+ languages run through a self-hosted Piston engine. stdout, stderr, images and tracebacks land in the console.

  • Python
  • Java
  • C
  • C++
  • C#
  • Go
  • Rust
  • Ruby
  • PHP
  • Kotlin
  • Swift
  • Lua
  • Scala
  • Perl
  • Haskell
  • Elixir
  • Dart
  • R
  • Julia
  • SQLite
  • Bash
  • PowerShell

08 questions

Compiling for the web — common questions

Questions about what compiles and what does not.

Is there really no such thing as an HTML compiler?

Correct, and it is worth being precise about. HTML is markup that a browser parses into a DOM — there is no compilation step and no output artefact. When people search for an HTML compiler they usually want one of three things: a preprocessor like Pug, a validator, or a preview. All three are here.

Does Pug compile to HTML in the browser?

Yes, the full language: mixins, includes, extends, iteration and interpolation. Errors report against the .pug file with a line number rather than producing malformed markup you then have to work backwards from.

Which Sass compiler is this?

Dart Sass, the reference implementation, compiled to WebAssembly and running client-side. Nesting, variables, mixins, @use and @forward all behave as they do locally.

Does it validate my HTML against the spec?

It catches the problems that actually cause bugs — unclosed tags, misnested elements, duplicate ids, missing required attributes — as you type. It is not a full W3C conformance checker, and for most work the difference does not matter.

Why does my page look wrong when the HTML looks right?

Usually parser error recovery. The browser never refuses malformed markup; it guesses. A <div> inside a <p> gets relocated, an unclosed tag re-nests everything after it. Comparing the rendered DOM against your source is the fastest way to see it.

Can I see the compiled output?

The generated HTML, CSS and JavaScript are what the preview runs, and every compile error quotes your source with a line and column. This is a working development loop rather than an inspection tool for its own intermediate files.

Does TypeScript get type-checked?

No. Annotations are stripped so the browser can run the result, which is the same trade a tsc-free dev loop makes locally. You get types for editing and completion; a full check belongs in CI.

Can I run a build tool like Vite or webpack?

No. Those are Node programs and a browser tab has no Node process. The compilers above cover what a dev-time build would do for a page like this; bundling and minification belong to a real build pipeline.

Do I need to install anything to use XCODX Studio?

No. XCODX Studio is a browser-based IDE — the editor, the compilers and the preview all run client-side. There is no download, no package manager to set up and no configuration file to write. Opening the editor is the whole setup step.

Where is my code stored?

In your own browser. Projects, file history and Git objects live in local storage on your device and are never uploaded to a server. That also means XCODX works offline: it is a Progressive Web App, so you can install it and keep coding with no connection.

Is XCODX Studio free?

Yes. The editor, the live preview, the 70+ language runtimes, npm installs, Git and the integrated terminal are all free to use.

Can I use XCODX on a tablet or Chromebook?

Yes. The editor is a responsive web app, so a Chromebook, an iPad or an Android tablet gets the same compilers and the same preview a laptop does — there is no desktop-only build, because there is no build to install at all. The layout adapts to the screen rather than shrinking a desktop UI into it.

Can I export a project or move it to my own machine?

Yes. Export the whole project as a zip with its folder structure intact and open it locally, or commit it and push. It is ordinary source on disk from that point on — no proprietary format and no lock-in, which is also why importing an existing repository from GitHub works the same way in reverse.

Nothing to install

Compile and validate now

Pug, SCSS and TypeScript compiled in milliseconds, with markup diagnostics as you type and no build tool to configure.

Open the compiler Read the docs