Fonts-weight descriptor

files/en-us/web/css/@font-face/font-weight/index.md

@@ -7,11 +7,16 @@ browser-compat: css.at-rules.font-face.font-weight
 
 {{CSSRef}}
 
-The **`font-weight`** CSS descriptor allows authors to specify font weights for the fonts specified in the {{cssxref("@font-face")}} at-rule. The {{cssxref("font-weight")}} property can separately be used to set how thick or thin characters in text should be displayed.
+The **`font-weight`** CSS {{cssxref("@font-face")}} descriptor enables authors to specify a single font weight, or a range of font weights, for the font specified in a {{cssxref("@font-face")}} at-rule. This is then used by the browser to select the appropriate font when a CSS rule sets a desired {{cssxref("font-weight", "font weight")}}.
 
-For a particular font family, authors can download various font faces which correspond to the different styles of the same font family, and then use the `font-weight` descriptor to explicitly specify the font face's weights. The values for the CSS descriptor is same as that of its corresponding font property.
+Typically, a developer will want to use fonts from a single font family in a range of different weights. With traditional, or _static_ fonts, a single font file contains characters from a font family in a specific weight and style: for example, "Helvetica bold italic". To enable displaying light, regular, bold, or extra-bold fonts when the `font-weight` property calls a specific weight, you can define multiple {{cssxref("@font-face")}} at-rules for the same family (all with the same {{cssxref("@font-face/font-family", "font-family")}} descriptor value), one for each weight or range of weights.
 
-There are generally limited weights available for a particular font family. When a specified weight doesn't exist, a nearby weight is used. Fonts lacking bold typeface are often synthesized by the user agent. To prevent this, use the {{cssxref('font-synthesis')}} shorthand property.
+To declare the font to be used for a range of font weights, declare a space-separated pair of font-weight values as the value for the `font-weight` descriptor. When CSS rules set a font weight by setting the {{cssxref("font-weight")}} property or the {{cssxref("font")}} shorthand property, the appropriate font will then be used.
+
+For example, if the descriptor is `font-weight: 400 600;`, when the property is `font-weight: 450` or `font-weight: 550`, that font will be use for that font-family.
+Whether the font is a static or a [variable font](/en-US/docs/Web/CSS/CSS_fonts/Variable_fonts_guide), the font matching the range will be used. In this case, if the font is a static font, `450` and `550` will apear the same. If the font is a variable font, the latter will be bolder.
+
+The descriptor is the same for all fonts, but the range you'll set for a variable font will generally be greater, possibly even `1 1000` to use the same font for all font weight property values.
 
 ## Syntax
 
@@ -21,26 +26,28 @@ font-weight: normal;
 font-weight: bold;
 font-weight: 400;
 
-/* Multiple Values */
+/* Defining a range */
 font-weight: normal bold;
 font-weight: 300 500;
 ```
 
-The `font-weight` property is described using any one of the values listed below.
-
 ### Values
 
+The `font-weight` descriptor takes one of the following forms:
+
+- The keyword `auto`.
+- A single `<font-weight-absolute>` value.
+- A pair of `<font-weight-absolute>` values, separated by a space.
+
+Each `<font-weight-absolute>` may be any one of the following:
+
 - `normal`
   - : Normal font weight. Same as `400`.
 - `bold`
   - : Bold font weight. Same as `700`.
 - `<number>`
   - : A {{cssxref("&lt;number&gt;")}} value between 1 and 1000, inclusive. Higher numbers represent weights that are bolder than (or as bold as) lower numbers. Certain commonly used values correspond to common weight names, as described in the [Common weight name mapping](#common_weight_name_mapping) section below.
 
-In earlier versions of the `font-weight` specification, the property accepts only keyword values and the numeric values 100, 200, 300, 400, 500, 600, 700, 800, and 900; non-variable fonts can only really make use of these set values, although fine-grained values (e.g. 451) will be translated to one of these values for non-variable fonts.
-
-CSS Fonts Level 4 extends the syntax to accept any number between 1 and 1000, inclusive, and introduces [Variable fonts](#variable_fonts), which can make use of this much finer-grained range of font weights.
-
 ### Common weight name mapping
 
 The numerical values `100` to `900` roughly correspond to the following common weight names:
@@ -80,20 +87,214 @@ People experiencing low vision conditions may have difficulty reading text set w
 
 ## Examples
 
-### Setting normal font weight in a @font-face rule
+### Selecting normal and bold fonts
+
+In this example we include two fonts, one normal weight, one bold weight, from the ["Fira Sans"](https://fonts.google.com/specimen/Fira+Sans) font family using two `@font-face` at rules. We set `font-weight` descriptors to match the weight of the fonts.
+
+After this, CSS rules can select the normal or the bold font for the "Fira Sans" family just by setting the {{cssxref("font-weight")}} property. Note that the {{htmlelement("strong")}} HTML element also selects the bold font, because by default `<strong>` elements have a CSS `font-weight` property value of `bold`.
+
+#### HTML
+
+```html
+<p class="one">Fira Sans, `normal` weight paragraph</p>
+<p class="two">Fira Sans, `bold` weight paragraph</p>
+<p><strong>Fira Sans, &lt;strong&gt; element (`bold`)</strong></p>
+```
+
+#### CSS
+
+```css
+@font-face {
+  font-family: "Fira Sans";
+  font-weight: normal;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-Regular.woff2");
+}
+
+@font-face {
+  font-family: "Fira Sans";
+  font-weight: bold;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-Bold.woff2");
+}
+
+body {
+  font-family: "Fira Sans", serif;
+  font-size: 2rem;
+}
+
+p.one {
+  font-weight: normal;
+}
+
+p.two {
+  font-weight: bold;
+}
+```
+
+#### Result
+
+{{embedlivesample("Selecting normal and bold fonts", "", 300)}}
+
+### Setting font-weight ranges
+
+This example demonstrates how authors can include multiple fonts for multiple font-weights (and font-styles), by including multiple `@font-face` declarations with the same `font-family` value. By setting the `font-weight` descriptors using ranges from 1 to 1000, in the rest of your stylesheets, you can declare a `font-weight` (or `font-style`), and know the appropriate font will be used.
+
+#### HTML
+
+```html
+<p class="one">This has a font weight of 100</p>
+<p class="three">This has a font weight of 300</p>
+<p class="four">This has a font weight of 400</p>
+<p class="five">This has a font weight of 500</p>
+<p class="seven">This has a font weight of 700</p>
+<p class="nine">This has a font weight of 900</p>
+```
+
+#### CSS
 
-The following finds a local Open Sans font or imports it, and allows using the font for normal font weights.
+We include four `@font-face` declarations for four different fonts from the `FireSans` font family, as seen in the previous example. Each declaration is set to a different range of font-weight values, but all use the same font name.
+
+The first declaration uses `FiraSans-Regular` and its associated `font-weight` range includes the entire possible range of font weight values.
+
+The other three declarations use the light, bold, and extra-bold versions of the font, and their associated `font-weight` ranges define subsets of the range as follows:
+
+- the light font is associated with the range 1-300
+- the bold font is associated with the range 500-700
+- the extra-bold font is associated with the range 700-1000
+
+The CSS [cascade](/en-US/docs/Web/CSS/Cascade) ensures that the three latter declarations override parts of the the range that was set in the `FiraSans-Regular` declaration.
 
 ```css
 @font-face {
-  font-family: "Open Sans";
-  src:
-    local("Open Sans") format("woff2"),
-    url("/fonts/OpenSans-Regular-webfont.woff") format("woff");
+  font-family: "Fira Sans";
+  font-weight: 1 1000;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-Regular.woff2");
+}
+
+@font-face {
+  font-family: "Fira Sans";
+  font-weight: 1 300;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-Light.woff2");
+}
+
+@font-face {
+  font-family: "Fira Sans";
+  font-weight: 500 700;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-Bold.woff2");
+}
+
+@font-face {
+  font-family: "Fira Sans";
+  font-weight: 700 1000;
+  src: url("https://mdn.github.io/shared-assets/fonts/FiraSans-ExtraBold.woff2");
+}
+
+body {
+  font-family: "Fira Sans", serif;
+  font-size: 2rem;
+}
+
+p.one {
+  font-weight: 100;
+}
+
+p.three {
+  font-weight: 300;
+}
+
+p.four {
   font-weight: 400;
 }
+
+p.five {
+  font-weight: 500;
+}
+
+p.seven {
+  font-weight: 700;
+}
+
+p.nine {
+  font-weight: 900;
+}
+```
+
+#### Result
+
+{{embedlivesample("Setting font-weight ranges", "", 400)}}
+
+The `seven` paragraph uses the extra bold font. While `font-weight: 700` matches both the `FiraSans-Bold` and `FiraSans-ExtraBold` declarations, as the FiraSans-ExtraBold is declared later, it overrides the `FiraSans-Bold` for that value.
+
+Similarly, the `100` and `300` both use the light font; although `FiraSans-Regular` and `FiraSans-Light` both include `300` in their ranges, `FiraSans-Light` is declared later. Alternatively, we could have declared `FiraSans-Regular` after `FiraSans-Light`, but we would need to change the `font-weight` descriptor range if we do so.
+
+### Setting a range for a variable font
+
+In this example we're using the `font-weight` descriptor to restrict the range of weights that can be set when using a variable font.
+
+We include a variable font, ["League Mono"](https://www.theleagueofmoveabletype.com/league-mono), using a single `@font-face` at-rule. We use a `font-weight: 300 700` value to effectively limit the range of weights that are available. If a CSS rule uses our "League Mono" font, then if it specifies a weight outside this range the weight it gets is clamped to the range.
+
+#### HTML
+
+We include a paragraph with `<output>` initially set to `400`, as that is the the default font-weight for unstyled paragraph text. This paragraph is nestled between two other paragraphs, so you can compare rendered versus declared font weight values.
+
+We include an {{htmlelement("input/range")}} of type `range`, nested in a {{htmlelement("label")}}, setting the `step` to `50`.
+
+```html
+<p>LeagueMono, font-weight: 300 (comparison)</p>
+<p id="example">LeagueMono, font-weight: <output>400</output> (example)</p>
+<p>LeagueMono, font-weight: 700 (comparison)</p>
+<label
+  >Change the font size:
+  <input type="range" min="50" max="1000" step="50" value="400" />
+</label>
+```
+
+#### CSS
+
+We set the `font-weight` descriptor range to `300 700`, clamping the variable font to that range.
+
+```css
+@font-face {
+  font-family: LeagueMono;
+  src: url("https://mdn.github.io/shared-assets/fonts/LeagueMono-VF.ttf");
+  font-weight: 300 700;
+}
+
+p {
+  font-family: LeagueMono, serif;
+  font-size: 1.5rem;
+}
+
+p:first-of-type {
+  font-weight: 300;
+}
+
+p:last-of-type {
+  font-weight: 700;
+}
+```
+
+#### JavaScript
+
+We include an event handler that updates the paragraph's `font-weight` property value, and updates the text to reflect the change:
+
+```js
+const text = document.querySelector("#example");
+const log = document.querySelector("output");
+const range = document.querySelector("input");
+
+range.addEventListener("change", () => {
+  text.style.fontWeight = range.value;
+  log.innerText = range.value;
+});
 ```
 
+#### Result
+
+{{embedlivesample("Setting a range for a variable font", "", "400")}}
+
+Change the font weight of the paragraph via the range.
+Note that the example paragraph does not get lighter than the `300` paragraph above it or bolder than the `700` paragraph below it; the font weight is clamped to the range defined by the `font-weight` descriptor.
+
 ## Specifications
 
 {{Specifications}}