Online HTML, CSS and JavaScript Editor
Build interactive pages with the HCODX HTML, CSS and JavaScript editor. Link your files, preview button behavior and inspect console output.
Give each file one job and one shared vocabulary
A working interaction is a small agreement between a document node, a visual state and an event handler.
A button can look perfect but do nothing if JavaScript queries an id that does not exist. A click handler can run correctly yet appear broken if CSS hides the updated result. Testing the three layers together catches these mismatches earlier than reading each file in isolation.
Start with one small behavior. Make sure the HTML works without a script, then add styling, then attach the event. That sequence gives you a known-good baseline if the final preview changes unexpectedly.
HTML
Use a real button element and a labelled output rather than a clickable generic div.
CSS
Keep interactive states visible, including focus-visible for keyboard users.
JavaScript
Query the correct element and attach the listener after the document is available.
Build a counter without a framework
The example is intentionally small: a button, an output, a focus style and one event listener.
Create the HTML and CSS first, then add the JavaScript. If you use separate files, link styles.css from the document and load app.js with defer so the elements exist before the query runs.
Click the button twice and confirm that the output reaches 2. Then use the console to investigate if the button does nothing: a null result from querySelector usually means the selector and markup disagree.
<!-- index.html -->
<link rel="stylesheet" href="styles.css">
<button id="add" type="button">Add one</button>
<output id="count" aria-live="polite">0</output>
<script src="app.js" defer></script>
/* styles.css */
#add:focus-visible { outline: 3px solid #2947ff; outline-offset: 3px; }
// app.js
const button = document.querySelector("#add");
const count = document.querySelector("#count");
let value = 0;
button.addEventListener("click", () => { count.textContent = String(++value); });Debug the join between files
Most first-project failures are connection mistakes, not difficult algorithms.
For a larger site, split related behavior into modules and use a build system when needed; the HCODX editor is a focused front-end workspace, not a server runtime.
What the three front-end files can run by themselves
HTML, CSS and JavaScript cover the page that reaches the browser; they do not automatically supply a backend.
Works in the preview
Markup, styling, DOM events, timers and other browser APIs allowed by the preview frame and browser permission model.
May depend on another origin
Fetching data from an API, embedding third-party scripts or loading remote assets depends on network availability, permissions and the remote service.
Needs another environment
PHP execution, private database queries, server-only secrets, build steps and authenticated backend routes are not produced by a front-end preview.
A form can validate locally without pretending to send data
The browser can check an email-shaped value and update the page. Saving or emailing that value is a different, server-backed task.
This example uses a real form and a labelled email input. The required and email attributes give the browser a first validation pass. When the input passes that check, the submit handler prevents navigation and writes an explicit local-only status message. There is no fetch request or storage call, so the form has not delivered anything to a person or database.
That distinction matters when testing generated interfaces: a success message can look like proof of submission even when no backend exists. If you later add an API, handle its response and failure states separately. Do not embed the API’s private credential in app.js; visitors can inspect front-end code.
<!-- index.html -->
<form id="email-check">
<label for="contact-email">Email</label>
<input id="contact-email" type="email" required>
<button type="submit">Check email</button>
<p id="status" role="status"></p>
</form>
<script src="app.js" defer></script>
// app.js
const form = document.querySelector("#email-check");
const status = document.querySelector("#status");
form.addEventListener("submit", event => {
event.preventDefault();
status.textContent = "Valid format. Nothing has been sent.";
});Test the states around the happy path
A control is not finished just because one click works with ideal input.
Use the keyboard
Tab to controls, submit a form with Enter and check that focus remains understandable after a result appears.
Try missing and malformed input
Confirm required fields and type constraints behave as intended. Write an error message if custom validation adds rules the browser does not know.
Check repeated actions
Click twice or submit twice. Ensure the handler does not duplicate output, attach itself again or leave stale status on screen.
Separate success from transport
When a backend is involved, do not show “sent” before the request has actually succeeded; display a recoverable failure state when it does not.
Three-language project questions
Make the button do something
Create the three files, run the counter and follow the first error if it fails.
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