HTML Checking for Large Sites

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 element, used to define the font face, size and color in previous versions of HTML, is no longer valid in HTML5.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>

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>

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

To query for the size of the viewport (or the page box on page media), the width, height and aspect-ratio media features should be used, rather than device-width, device-height and device-aspect-ratio, which refer to the physical size of the device regardless of how much space is available for the document being laid out. The device-* media features are also sometimes used as a proxy to detect mobile devices. Instead, authors should use media features that better represent the aspect of the device that they are attempting to style against.

The width media feature describes the width of the targeted display area of the output device. For continuous media, this is the width of the viewport including the size of a rendered scroll bar (if any).

In the following example, this media query expresses that the style sheet is only linked if the width of the viewport 768px maximum:

<link rel="stylesheet" media="only screen and (max-width: 768px)" href="styles.css">

To query for the size of the viewport (or the page box on page media), the width, height and aspect-ratio media features should be used, rather than device-width, device-height and device-aspect-ratio, which refer to the physical size of the device regardless of how much space is available for the document being laid out. The device-* media features are also sometimes used as a proxy to detect mobile devices. Instead, authors should use media features that better represent the aspect of the device that they are attempting to style against.

The width media feature describes the width of the targeted display area of the output device. For continuous media, this is the width of the viewport including the size of a rendered scroll bar (if any).

In the following example, this media query expresses that the style sheet is only linked if the width of the viewport is greater than 768px:

<link rel="stylesheet" media="only screen and (min-width: 768px)" href="styles.css">

The aspect-ratio CSS property allows you to define the desired width-to-height ratio of an element’s box. This means that even if the parent container or viewport size changes, the browser will adjust the element’s dimensions to maintain the specified width-to-height ratio. The specified aspect ratio is used in the calculation of auto sizes and some other layout functions.

The box’s preferred aspect ratio is the specified ratio of width / height. If height and the preceding slash character are omitted, height defaults to 1.

Here are some examples of this property:

aspect-ratio: 1 / 1;
aspect-ratio: 1;

/* Global values */
aspect-ratio: inherit;
aspect-ratio: initial;
aspect-ratio: revert;
aspect-ratio: revert-layer;
aspect-ratio: unset;

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 hexadecimal value for the color CSS property is not valid. It needs to have either 3 or 6 hexadecimal digits.

The color CSS property sets the foreground color value of an element’s text and text decorations, and sets the currentcolor value. currentcolor may be used as an indirect value on other properties and is the default for other color properties, such as border-color.

This property accepts colors in different formats, one of them being hexadecimal values. For example a pure red color can be expressed either with 3 hexadecimal digits or 6 hexadecimal digits:

color: #F00;
color: #FF0000;

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.

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