HTML Guide for square bracket

The attribute value in the href on the <a> element contains invalid characters.

HTML links require a valid URL as the value of the href attribute. Brackets ([, ]) are not allowed in attribute values.

Example of invalid usage:

<a href="mailto:[user@example.com]">Email Us</a>

Correct usage:

<a href="mailto:user@example.com">Email Us</a>

If you are using a template variable to create the links, be sure the template engine properly outputs a valid address in the final HTML.

Example using pseudocode for a templating engine (replace with your syntax as needed):

<a href="mailto:{{ address }}">Email Us</a>

Correct, fully rendered HTML after the template variable is replaced:

<a href="mailto:user@example.com">Email Us</a>

The src attribute contains square brackets ([ or ]) in the URL’s query string, which are not permitted in valid HTML URLs.

According to the HTML standard, attribute values such as URLs must conform to valid URI syntax. Unencoded square brackets are reserved characters in URI syntax and must be percent-encoded. Specifically, [ should be replaced with %5B and ] with %5D. This ensures the URL is correctly interpreted by browsers and validators. For example, a URL parameter like sort=[asc] should be coded as sort=%5Basc%5D.

Correct HTML Example:

<iframe src="/page?time=%5Btimestamp%5D"></iframe>

Always encode reserved characters in URLs when using them in HTML attribute values to ensure W3C compliance.