How do I test HTML online?

Paste your HTML into the editor at the top of this page. The checker lists any mistakes as you type, and the preview shows the page with its CSS and JavaScript running. Nothing to install, no sign-up.

How do I check if my HTML is valid?

Paste it into the tester and look at the Problems tab. If it says no errors, every tag is closed and properly nested and the page has its doctype and title. Errors come with the line number and how to fix them.

Is this the same as the W3C validator?

No. This checker catches the mistakes that break pages in practice (unclosed and mismatched tags, broken attributes, duplicate ids, a missing doctype or title) and explains them in plain English. The W3C Nu validator checks every rule in the HTML specification. Use it as well when you need strict, formal conformance.

Does the preview run JavaScript?

Yes. Your scripts, styles, fonts and images load just as they would in a normal page. The preview runs in a sandboxed frame, so the code can't reach your 99helpers account or anything else on the site.

Can I see how my HTML looks on a phone?

Yes. Switch the preview to phone width to see the page at the size of a typical phone screen. Add a viewport meta tag if it looks zoomed out. The checker warns you when it's missing.

Is my code uploaded anywhere?

No. Checking and previewing happen in your browser. Your code only leaves it if you choose to host it.

How do I host my HTML for free?

Press "Host this HTML free". Your code becomes a file (index.html, or any name you give it), you choose an address, and it's live at your-name.99helpers.site. A free account hosts pages up to 10 MB.

Can I change the page after it's hosted?

Yes. Edit it in the 99helpers code editor, or publish a new version to the same address. The link stays the same and earlier versions are kept.

Can I host a whole website, not just one HTML file?

Yes. Drop a folder or a .zip with your HTML, CSS, JavaScript and images, and the whole site goes live together.

Can I test an HTML file from my computer?

Yes. Click "Open file" and choose an .html file. It opens in the editor, where it's checked and previewed like pasted code.

Test HTML Online

Paste your HTML to check it for errors and see it running in a live preview. Happy with it? Host it free as a web page with its own link.

Check itErrors as you type

See itA live preview

Host itFree, on its own link

Loading the editor…

28 lines · 13 elements · 1.0 KB

Becomes index.html on its own link

Host your HTML free

Press "Host this HTML free" above and your code lands here as a file. Pick an address and it's live at your-name.99helpers.site. You can also drop an .html file, a folder or a .zip.

Drop an .html file to host it

Or use “Host this HTML free” above to host the code you tested

Choose a fileor drag it anywhere in this box

An HTML tester, checker and viewer in one

Test HTML online without setting anything up. Paste your code and three things happen at once: it's checked for errors, it's shown in a live preview, and it's ready to host as a real web page.

The checker reads your HTML the way a browser does and flags the mistakes that break pages: tags that are never closed, tags closed in the wrong order, a missing quote, duplicate ids. Each one comes with its line number and a short note on how to fix it.

When it looks right, one click turns your code into an HTML file and puts it online at your-name.99helpers.site, free. You get a link you can send to anyone.

How to test HTML in 4 steps

1. Paste your HTML

Paste or type your code into the editor, or open an .html file from your computer. A whole page or a small snippet both work.

2. Check the problems

Mistakes are listed as you type, each with its line number and how to fix it. Click one to jump straight to that line.

3. Watch the preview

The preview shows your page as a browser draws it, with your CSS and JavaScript running. Switch to phone width to see it on mobile.

4. Host it, if you like

Press "Host this HTML free" and your code becomes an index.html file, published at your-name.99helpers.site in seconds.

What the HTML checker looks for

Errors break the page or change how browsers read it. Warnings won't break anything, but are worth fixing.

Tags that are never closed

Error

A <div>, <span> or <section> without its closing tag, which drags the rest of the page into it.

Wrongly nested tags

Error

Tags closed in the wrong order, like <b><i>…</b></i>, and a block such as <div> inside a <p>.

Stray closing tags

Error

A </div> or </p> with nothing open to close, usually a sign of a missing or extra tag elsewhere.

Broken attributes

Error

A missing quote that swallows the rest of the tag, duplicate attributes, and tags missing their closing >.

Duplicate ids

Error

Two elements with the same id, which breaks links to #sections, labels and JavaScript.

Doctype and title

Error

A full page without <!DOCTYPE html> (quirks mode) or without a <title>.

Links in links, forms in forms

Error

Nesting that browsers quietly undo, so the page doesn't look the way the code says.

Images without alt text

Warning

Missing alt text hurts accessibility and image search.

Missing page basics

Warning

No lang on <html>, no <meta charset> and no viewport tag, so the page shows tiny on phones.

Obsolete and unknown tags

Warning

Tags like <center>, <font> and <marquee>, and misspelled element names.

Self-closing non-void tags

Warning

<div /> doesn't close itself in HTML. The browser ignores the slash and the div stays open.

A bare < in text

Warning

A < that isn't the start of a tag should be written as <.

Common HTML errors and how to fix them

The four mistakes that turn up most, and what the fixed code looks like. Try "Example with mistakes" in the tester to see them caught.

A missing quote

Wrong

<div class="hero>
  <h1>Welcome</h1>
</div>

Right

<div class="hero">
  <h1>Welcome</h1>
</div>

Without the closing quote, everything up to the next quote mark becomes part of the class, and the heading disappears.

Tags closed in the wrong order

Wrong

<p><b>30% off <i>this week</b></i></p>

Right

<p><b>30% off <i>this week</i></b></p>

Close tags in reverse order of opening: the last one opened is the first one closed.

A link that's never closed

Wrong

<li><a href="/shop">Shop now</li>
<li><a href="/faq">FAQ</a></li>

Right

<li><a href="/shop">Shop now</a></li>
<li><a href="/faq">FAQ</a></li>

An unclosed <a> keeps going, so the text after it turns into part of the same link.

A block inside a paragraph

Wrong

<p>Our menu:
  <div>Pizza</div>
</p>

Right

<div>Our menu:
  <div>Pizza</div>
</div>

A <p> can't contain a <div>, so the browser ends the paragraph early and the </p> is left over.

A live HTML preview that runs your code

The preview isn't a picture of your page. It is your page, drawn by your browser, with your stylesheets, fonts, images and JavaScript.

It updates as you type. Switch between desktop and phone width to check your layout on both.

Updates as you type

No run button to press.

Desktop and phone

Check both layouts in a click.

Sandboxed

Scripts run safely, away from your account.

From test to live: host your HTML file free

Most HTML testers stop at the preview. This one takes you all the way to a real page. Press Host this HTML free and your code is saved as a file (index.html unless you name it something else), you choose an address, and it's online at your-name.99helpers.site.

It's proper web hosting, not a shared playground link. Your page loads on its own address, works on every device and has HTTPS. Edit it later without changing the link, see how many people visit, and on Starter and Pro add a password or your own domain.

Who tests HTML here

HTML from ChatGPT, Claude or Gemini

Got a page from an AI chatbot? Paste it in to see whether it works, then host it on a real link in one click.

Learning HTML

Write a few tags, see what they do, and get a plain-English explanation when something's off.

Checking a page before it goes live

Catch an unclosed tag or a duplicate id before your visitors find it.

Quick prototypes and landing pages

Sketch a page, check it on desktop and phone widths, and send someone a link to it.

HTML validator vs. code playground vs. Test HTML

Three kinds of tool, side by side.

Feature	HTML validator	Code playground	99helpers Test HTML
Finds HTML errors	Every spec rule		The common ones
Explains how to fix them	Spec wording
Live preview
Phone-width preview		Some
Publish as a real web page		Often inside their site
Your own address			your-name.99helpers.site
No sign-up to test		Varies

Testing HTML: questions

How do I test HTML online?: Paste your HTML into the editor at the top of this page. The checker lists any mistakes as you type, and the preview shows the page with its CSS and JavaScript running. Nothing to install, no sign-up.
How do I check if my HTML is valid?: Paste it into the tester and look at the Problems tab. If it says no errors, every tag is closed and properly nested and the page has its doctype and title. Errors come with the line number and how to fix them.
Is this the same as the W3C validator?: No. This checker catches the mistakes that break pages in practice (unclosed and mismatched tags, broken attributes, duplicate ids, a missing doctype or title) and explains them in plain English. The W3C Nu validator checks every rule in the HTML specification. Use it as well when you need strict, formal conformance.
Does the preview run JavaScript?: Yes. Your scripts, styles, fonts and images load just as they would in a normal page. The preview runs in a sandboxed frame, so the code can't reach your 99helpers account or anything else on the site.
Can I see how my HTML looks on a phone?: Yes. Switch the preview to phone width to see the page at the size of a typical phone screen. Add a viewport meta tag if it looks zoomed out. The checker warns you when it's missing.
Is my code uploaded anywhere?: No. Checking and previewing happen in your browser. Your code only leaves it if you choose to host it.
How do I host my HTML for free?: Press "Host this HTML free". Your code becomes a file (index.html, or any name you give it), you choose an address, and it's live at your-name.99helpers.site. A free account hosts pages up to 10 MB.
Can I change the page after it's hosted?: Yes. Edit it in the 99helpers code editor, or publish a new version to the same address. The link stays the same and earlier versions are kept.
Can I host a whole website, not just one HTML file?: Yes. Drop a folder or a .zip with your HTML, CSS, JavaScript and images, and the whole site goes live together.
Can I test an HTML file from my computer?: Yes. Click "Open file" and choose an .html file. It opens in the editor, where it's checked and previewed like pasted code.

Test your HTML now

Paste your code at the top of this page. It's checked and previewed instantly, and you can host it free.

Test HTML More about HTML hosting