7 Free HTML Code Analyzers & Validators

HTML is the number one foundational document markup language.

HTML stands for Hypertext Markup Language – it is used to tag and mark documents with elements, attributes, and structure.

Without accurate and proper HTML, web pages would just be chaotic blocks of unformatted text. HTML guides the web browser to display the document as the designer or creator intends it to be displayed.

HTML is fairly simple and is one of the first languages web developers learn – though you cannot technically call it a ‘programming language’, as its sole intent is to markup documents.

Here, we’ll be providing a brief example of what HTML and validation is, overviewing some HTML validation tools, and explaining how HTML errors can affect your site’s SEO performance.

HTML Example

An HTML copy of the first portion of this document would be:

<h1>HTML Analyzers & Validators</h1>

<p>HTML is the number one foundational document markup language.</p>

Here, <h1> marks up the first line as a title and <p> marks up the first paragraph. Notice how the opening tags are different to the closed tags, which are closed with </.

There are around 100 total HTML markup tags – not too many in the grand scheme of things.

HTML and W3C

The W3C, World Wide Web Consortium, is a quasi-regulatory organization for the internet. They set out codes of best practices for web designers and developers to follow.

Over time, HTML has needed to adapt to different devices, browsers, and assistive technologies. HTML errors can cause readability issues, particularly on certain devices such as screen readers (which render text documents as speech for the blind or visually impaired).

Poorly constructed HTML can cause a number of issues that sabotage the readability and compatibility of your web pages.

This could even render the entire webpage unreadable or uncrawlable in exceptional circumstances.

The W3C has laid out best practices for HTML – if you follow those then, in theory, your site is more likely to be compatible cross-browser, device and technology – both now and in the future.

This is why we have HTML validators – they check the quality and ‘cleanliness’ of your HTML.

Why Do HTML Errors Occur?

HTML errors and bugs can result when you’re editing pages in HTML format and make a mistake, or when you use a buggy plugin or other tool to create and markup web pages.

Codeless web development apps and WYSIWYG editors mean that we generally don’t need HTML knowledge to create documents – the software does that for us.

But it’s not always perfect, and errors can occur, harming the technical quality of your webpages, in turn damaging SEO.

HTML Validation

To analyze HTML documents for errors and bugs, you can use an HTML validator.

HTML validators check and validate your HTML against W3C best practice guidelines. They’re simple online tools that load an HTML file or URL before parsing the HTML markup and analyzing it for errors and bugs versus W3C guidelines.

Some HTML guidelines are pretty trivial, e.g. W3C says you should use <strong> and not <b> to embolden text.

Both markup a document in a near-identical fashion, but it just so happens that some plugins or editors might use <b> instead of <strong> for whatever reason.

That said, HTML validation will route out genuine errors and bugs.

Some key examples include unclosed tags, incorrectly written tags, and formatting errors which cause compatibility issues, particularly with regards to tables.

In fact, you might have experienced dodgy HTML in the past when surfing a webpage that clearly doesn’t render properly, particularly old web pages that haven’t been optimized for mobile.

Dodgy HTML formatting issues can indirectly harm SEO, e.g. by forcing people off your site, or directly harm your SEO, e.g. by making pages uncrawlable.

Yoast SEO draws attention to an example of a Dutch news site whose homepage failed to crawl properly, and couldn’t be rendered in Google’s cache.

They were using an XMP tag, similar to the PRE tag, but instead of rendering tags internally, it outputs them instead. The XMP tag wasn’t closed correctly, so GoogleBot failed to render and crawl the page.

This is a pretty unique example, though, and you’re very unlikely to experience page-breaking HTML errors.

What Does Google Think About HTML Validation?

It’s worth remembering that W3C is not really affiliated with Google, and that you’re under no obligation to conform every component of your documents to their standard.

So what does Google say on the issue?

Google Search Central states:

The best way to make sure that your page looks the same in all browsers is to write your page using valid HTML and CSS, and then test it in as many browsers as possible. Clean, valid HTML is a good insurance policy,”

– Google.

Meanwhile, Google’s Matt Cutts admits that HTML validation is not an organic ranking factor, also stating that a minuscule percentage of sites have perfect HTML – and a large portion of global giant websites actually have pretty poor HTML on balance.

“Google does not penalize you if you have invalid HTML because there would be a huge number of webpages like that”

– Matt Cutts, Former Google Head of Webspam.

HTML, like many areas of SEO, is a grey area. You can leave messy code alone and hope it never matters – and it probably won’t unless there are serious errors – or you can strive long and hard to build the perfect HTML markup for every page on your site.

Largely, this comes down to time and resources. HTML gurus will whizz through validation, but it could still be time-consuming.

So What’s the Point in Validation?

Validation is non-essential but it’s still good practice, but the truth is, few SEOs take it particularly seriously unless they encounter errors.

W3C themselves do put forward a strong case for checking HTML, particularly as it will future proof your pages to new browser software and accessibility tools.

Debugging HTML errors and bugs are made simpler by HTML validation tools, and they can direct you towards serious issues. If you have knowledge of HTML then it should be fairly simple to use an HTML validator to check your code, but if not, then it can be pretty bewildering, especially if it throws up a lot of errors.

It’s also worth highlighting that HTML validation is not the only way to check page errors. SEO tools such as Semrush Site Audit and Surfer SEO offer a multitude of other on-page and technical SEO checkers that can crawl your site and will highlight errors that Google will see – and which could then affect your SEO.

You can also check for browser compatibility, Google highlights this themselves:

“Once you’ve created your web design, you should review your site’s appearance and functionality on multiple browsers to make sure that all your visitors are getting the experience you worked so hard to design.”

HTML Validator Tools

Without further ado, let’s suggest some HTML validator tools to plug your HTML in for markup analysis and validation.

W3 Validator

By W3C themselves, this validator is very easy to use via URL input, HTML file upload, or direct input. It handles all HTML, XHTML, and other web document formats.

Once you’ve input your URL and clicked validate, the tool will parse your HTML and display both errors and warnings. Errors are what you should focus on first, warnings usually related to unnecessary attributes or other markups that could be removed – but that’s usually unnecessary, or at least very low priority.

It is almost guaranteed that you’ll find some errors and likely many warnings. Try some competitor sites, friend’s sites, or other major sites and see how they compare – you’ll likely find many errors there also.

If you do find reams and reams of errors then don’t panic, and don’t rush into making any changes. Check your site on multiple browsers and use an SEO audit tool like Semrush SEO Audit to check the crawlability and technical SEO of your site.

If Google can crawl it and people can read it without issues, then you’re probably fine!

You can do your own research into errors – there are too many possibilities to describe but the likelihood is, if it’s a genuine issue then people will have discussed it many times already.

JSON Formatter HTML Validator

A powerful set of tools for formatting, converting, and validating HTML as well as JSON and CSS, the JSON Formatter HTML Validator works like any tool – you simply plugin your URL or upload a URL file.

You can hit ‘Run’ to render the HTML and Validate to check the HTML for errors. Again, the same caveats apply, and you’ll likely find errors – possibly many

The interface here isn’t as good as other HTML validator tools and doesn’t give you a detailed breakdown of errors and warnings, though it’s still fine if you have HTML knowledge and know what you’re looking at.

Freeformatter HTML Validator

Another simple HTML validator tool. The Freeformatter HTML Validator only lets you copy and paste HTML or upload an HTML file. You can choose from all ISO language codes for parsing HTML in different languages.

You’ll find that errors and warnings are clearly differentiated here, allowing you to look at common errors that appear on your pages and delve into further explanations online.

Infohound Tidy

This tool lets you check and validate HTML and then create and download a tidied copy. There are many options

Like other tools, it will display both warnings and errors. You can then tidy errors using the controls provided before downloading the tidied HTML.

DO NOT just copy HTML from your site into here and replace it with the tidied HTML, at least not without creating a duplicate to test it first, or using a backup.

Like with other HTML validators, it’s likely that many errors are negligible or pretty much meaningless – always research them before taking action.

Code Beautifier for HTML

Code beautification is slightly different from validation. With this tool, you can beautify your code, meaning deleting unnecessary components and minimizing down to what is needed.

This makes it easier to work with the code.

Again, this has a pretty niche use but will be useful for some


SonarSource is a professional suite of paid code analysis tools. They’re oriented towards creating highly robust, secure codes. The software helps tidy code but also identifies secure blackspots and other backdoors or other security issues.

For sure this is the most advanced HTML validator and analyzer around and each and every code or warning is broken down with an explanation of what’s going on.

For clients that demand perfectly validated error-free code, SonarSource is tough to beat.

What Do I Do If I Find HTML Errors?

It’s likely that you’ll find HTML errors when using a validation too.

Feature-rich pages that run lots of plugins will likely be the worst, whereas simple pages with little content, or just plenty of simple block text, will usually be fine or totally error-free.

The first thing to do is to check and see if any errors appear multiple times. Look them up to gain more info on what they mean and whether they matter – it might be jargon but you might find some simple-worded insight!

You can also consider hiring an HTML specialist on a freelance website. Checking your HTML should be easy for someone who knows HTML inside out and it’s likely that any serious errors can be fixed in seconds.

If all else fails, then you’ll need to check your pages using SEO audit tools and browser compatibility apps. If your pages load fine on every browser and device, and you sort technical SEO issues highlighted by an SEO audit, then you’ll probably be fine!

Make sure you monitor your site in Search Console, Google Analytics, or whatever other tools you use, and keep an eye for changes in traffic or other anomalies that could indicate more serious site bugs or UX problems.

The only other thing to do is to keep up-to-date with Google’s algorithm changes to make sure they never add HTML validation signals or other new rules that could lead to negative SEO results for sites with messy HTML – but this seems unlikely.


Clean HTML is a hot topic, with the purists arguing that fully clean, precise and valid HTML as validated by validation services is totally necessary for now and the future.

But, checking the HTML of many sites reveals a different story and Google themselves admit that ranking sites on their HTML would be likely destructive and antithetical to what they’re trying to do – i.e. serve the user.

But, therein lies the crux of the issue.

If HTML does affect user experience on your site, then it will affect SEO.

This is why HTML validation is a good way of tracking down HTML issues, and together with technical SEO audit tools, you can build a clean, fast and robust site.

I have been doing SEO in one form or another for the past 12 years. Currently, I have several clients and a small portfolio of websites. My aim with this site is to give beginners everything they need to get started in SEO.

SEO Administrator