HTML Checking for Large Sites

The id attribute of an HTML element, used to identify the element when linking, scripting or styling, must be unique in the whole document and must not contain whitespace.

Technically, in HTML5, the value for an id attribute may contain any character, except whitespace characters. However, to avoid inadvertent errors, only ASCII letters, digits, _, and - should be used and the value for an id attribute should start with a letter.

The pattern attribute is only allowed on input whose type is email, password, search, tel, text or url. Check the type used, and consider changing to one of the allowed types to enable pattern client-side validation.

The pattern attribute is a handy way of adding client-side validation on HTML forms without resorting to JavaScript. Check out this article to learn more about Input Pattern.

The value contact is not a valid option for the autocomplete attribute on an <input> element.

All HTML elements may have the hidden boolean attribute set. When specified on an element, it indicates that the element is not yet, or is no longer, relevant, so browsers won’t render it.

Boolean attributes don’t accept values, its presence represents the true value and its absence represents the false value.

<!-- This is invalid because the hidden attribute should not have a value set -->
<div hidden="false"></div>

<!-- The correct way to hide a div is like this -->
<div hidden>This will be hidden</div>

<!-- And to show the element, we just don't hide it -->
<div>This won't be hidden</div>

An <a> element has been found with an invalid href attribute, containing more than one # adjacent character.

The # is used to separate the fragment part of an URI (typically used to indicate a section within a document). For example, this is a valid link to a URI containing a fragment:

<a href="https://example.com/faqs#pricing">pricing</a>

The next example is invalid because it contains two adjacent # characters, so that the fragment part would be #pricing instead of pricing:

<a href="https://example.com/faqs##pricing">pricing</a>

The href attribute of an <a> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

The id attribute is used to identify a single element within a document, it’s not required, but if used it must be unique, and must not be an empty string.

IDs for HTML elements can’t be blank.

The src attribute on an <img> element contains an invalid character, that should be properly encoded as a URI percent-encoded character.

The only permitted value for the xmlns attribute is http://www.w3.org/2000/svg.

The namespace declaration for an <svg> element is provided by the xmlns attribute like this:

<svg xmlns="http://www.w3.org/2000/svg">
  <!-- more tags here -->
</svg>

Although using https in the URL looks like it’s going to be more secure, in fact this URL is not used to connect it to, only to declare the namespace.