標準メタデータ名

<meta> 要素を使用して、文書のメタデータを名前と値の組み合わせで提供することができ、 name 属性はメタデータ名を指定し、 content 属性は値を指定します。

HTML 仕様書は、以下の一連の標準メタデータ名を定義しています。

• application-name: ウェブページで動作しているアプリケーションの名前です。

  注:

  • ブラウザーはアプリケーションを識別するためにこれを使用することがあります。これはふつうアプリケーション名が含まれるものの、文書名や状態などの情報も含まれる <title> 要素とは異なります。
  • 単なるウェブサイトにはアプリケーション名を定義するべきではありません。
• author: この文書の著者です。
• description: ページの内容を短く正確に要約したものです。 Firefox や Opera など一部のブラウザーは、これをブックマークされたページの既定の説明として使用します。
• generator: このページを生成したソフトウェアの識別子です。
• keywords: ページのコンテンツに関連する語句で、カンマ区切りです。
• referrer: この文書からリクエストを送信するための HTTP の Referer ヘッダーの内容です。

  <meta name="referrer"> の content 属性の値

  no-referrer	HTTP の Referer ヘッダーを送信しません。
  origin	文書のオリジンを送信します。
  no-referrer-when-downgrade	現在のページと同等の安全性の URL (HTTP(S)→HTTPS) にはリファラーとして URL 全体を送信しますが、安全性が低い URL (HTTPS→HTTP) には送信しません。これは既定の動作です。
  origin-when-cross-origin	同一オリジンへのリクエストでは URL 全体 (引数を除く) を送信しますが、他の場合はオリジンのみ送信します。
  same-origin	同一オリジンにはリファラーを送信しますが、オリジン間リクエストにはリファラーを含めません。
  strict-origin	安全性が同等とみられる宛先 (HTTP(S)→HTTPS) に対しては、リファラーとして文書のオリジンのみを送信しますが、安全性が劣る宛先 (HTTPS→HTTP) には送信しません。
  strict-origin-when-cross-origin	同一オリジンへのリクエストでは、 URL 全体 (引数を除く) を送信します。現在のページと安全性が同等であると見られる宛先 (HTTP(S)→HTTPS) にはオリジンを送信します。それ以外は、リファラーを送信しません。
  unsafe-URL	同一オリジンまたはオリジン間リクエストで、 URL 全体 (引数を除く) を送信します。

  注:

  • Dynamically inserting <meta name="referrer"> (with document.write() or appendChild()) makes the referrer behaviour unpredictable.
  • When several conflicting policies are defined, the no-referrer policy is applied.
• theme-color (en-US): indicates a suggested color that user agents should use to customize the display of the page or of the surrounding user interface. The content attribute contains a valid CSS <color>.

The CSS Color Adjustment specification defines the following metadata name:

• color-scheme: specifies one or more color schemes with which the document is compatible.

  The browser will use this information in tandem with the user's browser or device settings to determine what colors to use for everything from background and foregrounds to form controls and scrollbars. The primary use for <meta name="color-scheme"> is to indicate compatibility with—and order of preference for—light and dark color modes.

  The value of the content property for color-scheme may be one of the following:

  normal

  The document is unaware of color schemes and should simply be rendered using the default color palette.

  [light | dark]+

  One or more color schemes supported by the document. Specifying the same color scheme more than once has the same effect as specifying it only once. Indicating multiple color schemes indicates that the first scheme is preferred by the document, but that the second specified scheme is acceptable if the user prefers it.

  only light

  Indicates that the document only supports light mode, with a light background and dark foreground colors. By specification, only dark is not valid, because forcing a document to render in dark mode when it isn't truly compatible with it can result in unreadable content; all major browsers default to light mode if not otherwise configured.

  For example, to indicate that a document prefers dark mode but does render functionally in light mode as well:

  <meta name="color-scheme" content="dark light">

  This works at the document level in the same way that the CSS color-scheme (en-US) property lets individual elements specify their preferred and accepted color schemes. Your styles can adapt to the current color scheme using the prefers-color-scheme CSS media feature.

The CSS Device Adaptation specification defines the following metadata name:

• viewport: gives hints about the size of the initial size of the viewport. Used by mobile devices only.

  Values for the content of <meta name="viewport">

  値	Possible subvalues	解説
  width	A positive integer number, or the text device-width	Defines the pixel width of the viewport that you want the web site to be rendered at.
  height	A positive integer, or the text device-height	Defines the height of the viewport. Not used by any browser.
  initial-scale	A positive number between 0.0 and 10.0	Defines the ratio between the device width (device-width in portrait mode or device-height in landscape mode) and the viewport size.
  maximum-scale	A positive number between 0.0 and 10.0	Defines the maximum amount to zoom in. It must be greater or equal to the minimum-scale or the behaviour is undefined. Browser settings can ignore this rule and iOS10+ ignores it by default.
  minimum-scale	A positive number between 0.0 and 10.0	Defines the minimum zoom level. It must be smaller or equal to the maximum-scale or the behaviour is undefined. Browser settings can ignore this rule and iOS10+ ignores it by default.
  user-scalable	yes or no	If set to no, the user is not able to zoom in the webpage. The default is yes. Browser settings can ignore this rule, and iOS10+ ignores it by default.
  viewport-fit	auto, contain or cover	The auto value doesn’t affect the initial layout viewport, and the whole web page is viewable. The contain value means that the viewport is scaled to fit the largest rectangle inscribed within the display. The cover value means that the viewport is scaled to fill the device display. It is highly recommended to make use of the safe area inset variables to ensure that important content doesn't end up outside the display.

  Notes:

  • Though unstandardized, this declaration is respected by most mobile browsers due to de-facto dominance.
  • The default values may vary between devices and browsers.
  • To learn about this declaration in Firefox for Mobile, see this article.

  ビューポートの拡大縮小についてのアクセシビリティの考慮

  Disabling zooming capabilities by setting user-scalable to a value of no prevents people experiencing low vision conditions from being able to read and understand page content.
  • MDN Understanding WCAG, Guideline 1.4 explanations
  • Understanding Success Criterion 1.4.4 | W3C Understanding WCAG 2.0

  関連情報

  The @viewport CSS at-rule.

The WHATWG Wiki MetaExtensions page contains a large set of non-standard metadata names that have not been formally accepted yet; however, some of the names included there are already used quite commonly in practice — including the following:

• creator: the name of the creator of the document, such as an organization or institution. If there are more than one, several <meta> elements should be used.
• googlebot, a synonym of robots, is only followed by Googlebot (the indexing crawler for Google).
• publisher: the name of the document's publisher.
• robots: the behaviour that cooperative crawlers, or "robots", should use with the page. It is a comma-separated list of the values below:

  Values for the content of <meta name="robots">

  値	解説	Used by
  index	Allows the robot to index the page (default).	All
  noindex	Requests the robot to not index the page.	All
  follow	Allows the robot to follow the links on the page (default).	All
  nofollow	Requests the robot to not follow the links on the page.	All
  all	Equivalent to index, follow	Google
  none	Equivalent to noindex, nofollow	Google
  noarchive	Requests the search engine not to cache the page content.	Google, Yahoo, Bing
  nosnippet	Prevents displaying any description of the page in search engine results.	Google, Bing
  noimageindex	Requests this page not to appear as the referring page of an indexed image.	Google
  nocache	Synonym of noarchive.	Bing

  Notes:

  • Only cooperative robots follow these rules. Do not expect to prevent e-mail harvesters with them.
  • The robot still needs to access the page in order to read these rules. To prevent bandwidth consumption, use a robots.txt file.
  • If you want to remove a page, noindex will work, but only after the robot visits the page again. Ensure that the robots.txt file is not preventing revisits.
  • Some values are mutually exclusive, like index and noindex, or follow and nofollow. In these cases the robot's behaviour is undefined and may vary between them.
  • Some crawler robots, like Google, Yahoo and Bing, support the same values for the HTTP header X-Robots-Tag; this allows non-HTML documents like images to use these rules.