You got it wrong, the terrible idea is `pyxy-org/pyxy`

>> invalid TypeScript.

> If you don't happen to have react, preact or a similar front end library, and a appropriate bundler, it is invalid TypeScript.

> Not true, you can compile it to HTML on the backend or even statically too.

I had to test your assertion that it compiles to HTML. I remember it being very invalid typescript without react scafolding. So I tested it without the react scafolding:

    # mise use node@24
    # echo '{}' >package.json
    # npm i --save typescript
    # echo 'function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
    ' >hello.tsx
    # : a test to check if we have a working typescript compiler
    # tsc hello.tsx

    error TS5112: tsconfig.json is present but will not be loaded if files are specified on commandline. Use '--ignoreConfig' to skip this error.

    # echo '{"compilerOptions":{"jsx":"react"}}' >tsconfig.json
    # tsc

    hello.tsx:1:58 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                            ~~~~~~~~~~~~~~~~~~

    hello.tsx:1:59 - error TS2874: This JSX tag requires 'React' to be in scope, but it could not be found.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                ~~~

    hello.tsx:1:77 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                ~~~~

    hello.tsx:1:78 - error TS2874: This JSX tag requires 'React' to be in scope, but it could not be found.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                ~~

    hello.tsx:1:95 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                                    ~~~~~

    hello.tsx:1:101 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                                        ~~~

    hello.tsx:1:102 - error TS2874: This JSX tag requires 'React' to be in scope, but it could not be found.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                                        ~

    hello.tsx:1:123 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                                                                ~~~~

    hello.tsx:1:128 - error TS7026: JSX element implicitly has type 'any' because no interface 'JSX.IntrinsicElements' exists.

    1 function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }
                                                                                                                                    ~~~~~~

    Found 9 errors in the same file, starting at: hello.tsx:1

    # cat hello.js

    function Greeting({ name }) { return (React.createElement("div", { class: "card" }, " ", React.createElement("h1", null, "Hello, ", name, "!"), " ", React.createElement("p", null, "Welcome to my site."), " ")); }

    # : sanity check
    # mise use deno
    # deno hello.tsx

    # : no output

    # echo 'function Greeting({ name }: { name: string }) { return ( <div class="card"> <h1>Hello, {name}!</h1> <p>Welcome to my site.</p> </div> ); }; Greeting({ name: "world!" });' >hello2.tsx

    # deno hello2.tsx

    error: Uncaught (in promise) ReferenceError: React is not defined
        at Greeting (file:////hellotsx/hello.tsx:1:49)
        at file:////hellotsx/hello.tsx:1:141