HTML Guide
Due to an issue in the W3C validator, this is identified as an error but it’s not. This issue has been notified, and we’ll update our validator as soon as there’s a fix.
According to the WAI-ARIA 1.2 spec:
The aria-setsize property defines the number of items in the current set of listitems or treeitems. Not required if all elements in the set are present in the DOM.
Authors MUST set the value of aria-setsize to an integer equal to the number of items in the set. If the total number of items is unknown, authors SHOULD set the value of aria-setsize to -1.
The alert role can be used to tell the user an element has been dynamically updated. Screen readers will instantly start reading out the updated content when the role is added. The element <ul> doesn’t accept this kind of role, consider using other element like <p> or <div>.
The alert role is used to communicate an important and usually time-sensitive message to the user. When this role is added to an element, the browser will send out an accessible alert event to assistive technology products which can then notify the user about it. The alert role is most useful for information that requires the user’s immediate attention.
The aria-expanded attribute can only be true, false, or undefined.
This attribute indicates whether a grouping element is expanded or collapsed.
The width and height attributes on <img> elements expect a digit to specify the dimension in pixels. It should not contain units, letters or percent signs.
You can achieve this using CSS instead, for example:
<!-- Invalid syntax, the height attribute expects only digits -->
<img src="photo.jpg" alt="cat" height="auto" />
<!-- Valid syntax using CSS -->
<img src="photo.jpg" alt="cat" style="height: auto" />
The sizes attribute specifies the size of the image when it is displayed on different devices.
The error message is saying that the value auto is not a valid value for the sizes attribute.
To fix this issue, you need to replace the value auto with a valid size. You can use a width descriptor or a media query to specify the size for different device widths.
Here’s an example of using a width descriptor:
<img src="example.jpg" sizes="(max-width: 600px) 100vw, 50vw" />
This example sets the size of the image to 100% of the viewport width when the device width is less than or equal to 600px, and 50% of the viewport width for larger device widths.
Alternatively, you can remove the sizes attribute altogether and let the browser decide the best size for the image based on the viewport size.
<img src="example.jpg" />
If you do this, the browser will use the default sizes value of 100vw and will scale the image accordingly.
An element like <h1>, <h2>, etc., used to define a heading, does not accept the button role.
The following HTML code is invalid because the <h2> element can’t have role="button"
<h2 role="button">Some heading</h2>
Instead, you can nest the <h2> inside a <div> with that role. In this case however, browsers automatically apply role presentation to all descendant elements of any button element as it is a role that does not support semantic children.
<div role="button">
<h2>Some heading</h2>
</div>
A <li> element, used to define a list item, does not accept the button role.
This HTML code is invalid because the <li> elements can’t have role="button":
<ul>
<li role="button">One</li>
<li role="button">Two</li>
</ul>
The value cache-control is no longer a valid pragma directive. A better alternative is defining cache-control as an HTTP header.
The value cleartype is not valid for the property http-equiv on a meta tag.
The value company is not a valid option for the autocomplete attribute on an <input> element. You may use the organization value instead, as it can be used for “company name corresponding to the person, address, or contact information in the other fields associated with this field”.