Transpiling ts to js is as simple as loading the typescript.js file from typescript repo or npm, and using it's window.ts.transpile(tsCode)

JSFiddle

<script src="https://unpkg.com/typescript@latest/lib/typescript.js"></script>
<script>
const tsCode = 'let num: number = 123;';
const jsCode = window.ts.transpile(tsCode);
document.write(jsCode);
</script>

Outputs:

var num = 123;

You can also pass the ts compiler options object as second argument like:

ts.transpile(tsxCode, { jsx: "react", target: "es2015" });

Though for meaning of values you'll likely have to dive into the source code.

Since this question got re-opened (just as planned >:D), I'm also re-posting my comment here.

@basarat's solution was great; even though his lib is outdated and abandoned, it helped me a lot in writing another, self-sufficient modern lib with support for sub-dependencies: ts-browser

Usage: (given you use relative paths in all your ts files)

<!-- index.html -->
<script type="module">
    import {loadModule} from 'https://klesun.github.io/ts-browser/src/ts-browser.js';
    // language=file-reference
    const entryScriptPath = './index.ts';
    loadModule(entryScriptPath).then(indexModule => {
        return indexModule.default(document.getElementById('composeCont'));
    });
</script>

// index.ts
import {makePanel} from './utils/SomeDomMaker'; // will implicitly use file with .ts extension

export default (composeCont) => {
    composeCont.appendChild(makePanel());
};