HTML Checking for Large Sites

The <table> element does not accept a height attribute. Use CSS instead.

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"  />

Learn more:

• MDN: the img element

The attributes width and height of <img> elements expect a non-negative integer, so an empty string is not allowed. Either define the correct dimension, or remove this attribute.

Learn more:

• HTML Spec: images

The attributes width and height on <img> elements define the dimensions of the image in CSS pixels, and expect a non-negative integer.

Learn more:

• HTML Spec: dimension attributes

The <iframe> element, used to embed another document inside the current document, accepts both attributes width and height which must be valid non-negative integers. Percentages are not allowed for these attributes.

Check the iframe spec.

<img> elements accept a width attribute to specify the size in pixels. This value can only be an integer, it should not contain units or %. If you need to specify a percentage width, you can do that with CSS:

<img src="photo.jpg" alt="red car" style="width:100%;">

The width and height attributes on <img> and <iframe> elements expect a digit to indicate the number of pixels. Ensure that this attribute contains only digits.

For example:

<!-- This is invalid because width is not a digit -->
<img width="225px" height="100px" alt="cat" src="cat.jpg" />

<!-- This is valid -->
<img width="225" height="100" alt="cat" src="cat.jpg" />

A CSS definition for background-image could not be understood by the parser. Check its definition to ensure that it’s well formed and that it contains an appropriate value.

The value on the display property is not valid.

The display CSS property sets whether an element is treated as a block or inline element and the layout used for its children, such as flow layout, grid or flex.

Learn more:

• MDN: CSS display property

The specified CSS filter is not a standard one, and may only work in some browsers.

Check out the Filter Effects W3C Working Draft

font-display isn’t a CSS property, it’s a descriptor for use with the @font-face at-rule.

Learn more:

• Can I Use… font-display
• MDN: font-display
• MDN: At-rules
• W3C Validator issue on font-display

This issue is a false positive by the W3C validator, fixed in the latest versions of Nu Validator.

The value revert is indeed a valid value for the CSS property font-size.

Learn more:

• MDN: Revert
• MDN: Font-size

The @import CSS rule can be used to import a style sheet into another style sheet. It must appear at the top of the document, and after any @charset declaration.

Learn more:

• MDN: CSS import

The max-width CSS property sets the maximum width of an element. It prevents the used value of the width property from becoming larger than the value specified by max-width.

This property can express a value in different units like px, em, % or ch, and keyword values, but auto is not an allowed value.

Some examples of valid values for max-width:

/* <length> value */
max-width: 3.5em;

/* <percentage> value */
max-width: 75%;

/* Keyword values */
max-width: none;
max-width: max-content;
max-width: min-content;
max-width: fit-content(20em);

/* Global values */
max-width: inherit;
max-width: initial;
max-width: revert;
max-width: unset;

Learn more:

• MDN: CSS max-width

The value specified for the min-height CSS attribute is not valid.

Learn more:

• MDN: CSS min-height

An invalid CSS property is being used. Properties starting with mso- are commonly defined by Microsoft products like Office and Outlook.

These properties, like mso-spacerun, mso-fareast-font-family, mso-bidi-font-weight, mso-margin-bottom-alt, mso-margin-top-alt and others starting with mso- are not standard CSS properties.

Padding properties, unline margin properties, don’t accept negative values. Read more about the CSS box model

CSS styles could not be parsed, check the indicated line to find what caused the parser to fail. Common causes are unclosed curly brackets or comments, missing semicolons or unexpected or empty properties.

Read more about CSS: Cascading Style Sheets syntax

The value break-word for the property word-break is deprecated, you should replace it with a valid value.

According to the CSS Text Module Level 3 spec:

For compatibility with legacy content, the word-break property also supports a deprecated break-word keyword. When specified, this has the same effect as word-break: normal and overflow-wrap: anywhere, regardless of the actual value of the overflow-wrap property.

Learn more:

• MDN: word-break
• CSS Text Module Level 3: word-break property
• W3C Validator issue on break-word

The value specified for the transform CSS attribute is not valid.

The transform CSS property lets you rotate, scale, skew, or translate an element. It modifies the coordinate space of the CSS visual formatting model.

Learn more:

• MDN: CSS transform
• MDN: CSS visual formatting model

The ::file-selector-button has not been recognized by the W3C Validator. This seems to be a bug in the W3C Validator, which has already been reported.

Learn more:

• MDN: file-selector-button
• MDN: pseudo elements
• W3C: file-selector pseudo element

CSS can’t be properly processed because a computed style is missing a value it depends on.

A CSS definition for an X property could not be understood by the parser. Check it to ensure that it’s well formed and that it contains an appropriate value.

The specified CSS property is not valid.

Learn more:

• Can I Use…

When specifying a numeric value for a CSS property, remember to always include the units, like 3px or 3em. Only the 0 value can go without units, and instead of 0px you can just use 0.

Learn more:

• MDN: CSS values and units

<td> elements no longer accept an align attribute. This can be achieved using CSS like this:

<td style="text-align:center;">content</td>

The <big> tag is now obsolete. It was used to increase the size of text, you can do that using CSS instead. For example:

<p>Now this is <span style="font-size: larger;">big</span></p>

Read more:

• W3C: obsolete features

<img> tags no longer accept a border attribute. This can be defined using CSS instead, for example:

<img src="..." alt="..." style="border:0;" />

The <center> tag is no longer valid in HTML, you should use CSS instead, for example:

<p style="text-align:center">this text will be centered</p>

The <font> element, used to define the font face, size and color in previous versions of HTML, is no longer valid in HTML5. Instead, you should rely on CSS styles.

For example, you can define a font size of 12px, bold text with inline styles like this:

<p style="font-size: 12px; font-weight: bold;">some text</p>

Learn more about CSS fonts:

• MDN: CSS font

The <tt> tag, used in HTML4 to apply a monospaced (fixed width) font to the text, was deprecated in HTML5. Instead, you should use CSS to apply the desired font.

Example, instead of this deprecated code:

<tt>This is deprecated</tt>

You can define a monospaced text using font-family:

<span style="font-family: monospace;">This is monospaced text</span>

Learn more:

• MDN: font-family

The HTML <style> element contains style information for a document, or part of a document, defined in CSS. This element does not need the type attribute anymore, so it should be omitted.

For example, this style defines that <p> elements should be in red color.

<style type="text/css">
p {
  color: red;
}
</style>
<p>This text will be red.</p>

But, the type attribute is not used anymore, so we can just use this:

<style>
p {
  color: red;
}
</style>
<p>This text will be red.</p>

Read more:

• MDN: The Style information element
• WordPress plugin: Type Attribute Warnings Removal

<td> elements no longer accept a valign attribute. This can be achieved using CSS like this:

<td style="vertical-align:middle;">content</td>

Both <table> and <td> elements no longer accept a width attribute. Instead, you should use CSS as in this example:

<table style="width:100%;">
  <tr>
    <td style="width:50px;">Name</td>
  </tr>
</table>

You’re using an attribute X that is no longer valid for element Y, but you can use CSS to achieve the same effect.