HTML Guide

The font-stretch property in CSS defines the relative width of the font, and is used to make the text narrower or wider. The value bold is not a valid value for font-stretch. Instead, you should use the font-weight property to set the boldness of the font.

Here’s an example of how to use the font-weight property to set the text to bold:

<p style="font-weight: bold;">This text is bold.</p>

Alternatively, you can use a CSS stylesheet to apply the font-weight property to multiple elements:

<style>
  p { font-weight: bold; }
  h1 { font-weight: bolder; }
</style>

<p>This text is bold.</p>
<h1>This heading is even bolder.</h1>

The font-style CSS property sets whether a font should be styled with a normal, italic, or oblique face from its font-family.

Here are examples of valid font-style values:

font-style: normal;
font-style: italic;
font-style: oblique;
font-style: oblique 10deg;

/* Global values */
font-style: inherit;
font-style: initial;
font-style: revert;
font-style: revert-layer;
font-style: unset;

A common issue is trying to use font-style to define the size, when font-size should have been used instead, for example:

/* Invalid */
font-style: 1.2em;

/* Valid */
font-size: 1.2em;

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

The attribute font-weight can be used on SVG text elements like text but not on g container elements, and none is not a valid value.

The font-weight attribute refers to the boldness or lightness of the glyphs used to render the text, relative to other fonts in the same font family.

This attribute can be used with the SVG elements text, textPath, tref and tspan, but on on g elements.

Its allowed values are normal, bold, bolder, lighter, or a number. The value none is not valid for this attribute.

Here’s an example:

<svg viewBox="0 0 200 30" xmlns="http://www.w3.org/2000/svg">
  <text y="20" font-weight="normal">Normal text</text>
  <text x="100" y="20" font-weight="bold">Bold text</text>
</svg>

Alternatively, SVG text elements can also be stilyzed using CSS, like so:

<svg viewBox="0 0 200 30" xmlns="http://www.w3.org/2000/svg">
  <text y="20" style="font-weight:normal">Normal text</text>
  <text x="100" y="20" style="font-weight:bold">Bold text</text>
</svg>

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.