HTML - Validation

Types of HTML Validation

Syntax Validation

Syntax validation checks your HTML code for proper syntax and structure. This involves making sure that all tags are properly opened and closed, attributes are correctly formatted, and the overall structure of the document follows the HTML specification. Common syntax errors include missing closing tags, incorrect nesting of elements, and using invalid characters in tag names or attributes. By running your HTML through a syntax validator, you can catch these errors early and fix them before they cause problems in your web pages.

Example: HTML Code with Extra Spaces

<p>This    is   a   paragraph   with    extra   spaces.</p>

When a browser renders this code, it will display the text as:

This is a paragraph with extra spaces.

Example: Mismatched Tags

<p>This is a paragraph.</div>

In this case, the opening <p> tag is closed with a </div> tag, which is incorrect. The right way to close the paragraph is:

<p>This is a paragraph.</p>

Accessibility Validation

Accessibility validation makes sure that your HTML is accessible to users with disabilities. This includes making sure that your content can be read by screen readers, that your images have alternative text descriptions, and that your page can be navigated using only a keyboard. The Web Content Accessibility Guidelines (WCAG) provide guidelines for creating accessible HTML, including recommendations for using semantic markup, providing text alternatives for non-text content, and designing for keyboard navigation.

SEO Validation

SEO validation optimizes your HTML for search engines. This involves making sure that your pages have appropriate title tags, header tags, and meta descriptions as well as ensuring that your content is keyword-rich and relevant to your target audience. Some best practices for SEO-friendly HTML include using descriptive titles and headers with keywords, providing alt text for images, and using schema markup to provide additional context to search engines.

HTML Validation Tools

W3C Markup Validation Service

The W3C Markup Validation Service is a free online tool that checks your HTML code for conformance to web standards. Developed by the World Wide Web Consortium (W3C), this service helps web developers create standards-compliant websites. To use the W3C Markup Validation Service, enter your web page's URL or upload your HTML file. The tool will analyze your code and provide a report of any errors or warnings, along with suggestions for fixing them.

Browser Developer Tools

Most modern web browsers come with built-in developer tools that can help you validate your HTML code. These tools typically include an "Elements" or "Inspector" tab that allows you to view and edit the HTML structure of a web page in real-time. In Google Chrome, you can right-click on any element and select "Inspect" to open the Chrome Developer Tools. From there, you can check for any HTML errors, such as missing or mismatched tags, and fix them directly in the browser. Other popular browsers like Firefox, Safari, and Microsoft Edge also have similar features in their developer tools.

IDE and Text Editor Plugins

Many popular Integrated Development Environments (IDEs) and text editors offer HTML validation plugins that can check your code as you type. These plugins catch errors early in the development process before you run your code in a browser. Some examples of these plugins include:

Example: HTML Tidy for Sublime Text

<p>This is a paragraph.</p>

This plugin uses the HTML Tidy library to check and clean up your HTML code.

Example: HTML Validator for Atom

<p>This is a paragraph.</p>

This plugin validates your HTML against W3C standards.

Example: HTML Validation for Visual Studio Code

<p>This is a paragraph.</p>

This extension uses Nu Html Checker to validate your files.

Using these plugins helps you write cleaner, more valid HTML code and avoid common mistakes that can cause problems later on.

Fixing Validation Errors

Fixing validation errors is an important step in creating clean, standards-compliant HTML code. Some common validation errors include missing or mismatched tags, incorrect attribute values, and using outdated elements. To fix these errors, start by running your HTML through a validation tool like the W3C Markup Validation Service. This will give you a list of errors and their locations in your code.

One common error is missing closing tags. If you have an opening <p> tag but forget to include the closing </p> tag, the validator will flag this as an error. To fix it, simply add the missing closing tag.

Another common error is using incorrect attribute values.

Example: Invalid href attribute value

<a href="invalid url">Link</a>

The validator will catch this error. To fix it, make sure you use a valid URL format:

Example: Correct href attribute value

<a href="https://www.example.com">Link</a>

Using outdated elements is another source of validation errors. Outdated elements are old HTML tags that have been replaced by newer elements.

Example: Deprecated font tag

<font color="red">This is red text.</font>

The validator will warn you that this tag is no longer recommended. To fix this, replace the outdated tag with a more modern equivalent and use CSS for styling:

Example: CSS for styling text

<span style="color: red;">This is red text.</span>

To avoid validation errors in the first place:

  • Always include the <!DOCTYPE html> declaration at the beginning of your HTML files.
  • Use lowercase letters for tag names and attribute values.
  • Quote attribute values.
  • Close all tags that require closing like <p> and <li>.
  • Nest elements properly and don't overlap closing tags.
  • Use semantic elements like <header>, <nav>, and <article> to give meaning to your content.

By fixing validation errors and following best practices for writing HTML code can help create web pages that work consistently across browsers and devices.