當PageSpeed Insights檢測到您的網(wǎng)頁未指定視口，或者指定的視口不適合不同設(shè)備時，就會觸發(fā)此規(guī)則。

Viewport can control how web pages are displayed on mobile devices. If no viewport is specified, the mobile device renders the Web page to the width of a typical desktop screen and adjusts the Web page to fit the screen size. By setting the viewport, you can control the width and scaling of web pages on different devices.

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Left: Web page with no meta-viewport specified.

經(jīng)過優(yōu)化可在移動設(shè)備上正常顯示的網(wǎng)頁應在文檔標頭中包含元視口，指定如下內(nèi)容：width=device-width, initial-scale=1.

• Hardware pixel: The physical pixel of the display. For example, the iPhone 5 comes with a screen with 640 horizontal hardware pixels.
• Device-independent pixels (dip): under normal visual distance, it conforms to uniform參考像素的設(shè)備像素比例，此像素在所有設(shè)備上大小幾乎相同。iPhone 5的設(shè)備無關(guān)像素寬度為320。
• CSS像素：用于頁面布局的單位，由視口控制。樣式的像素尺寸（例如width: 100px) is specified in units of CSS pixels. The ratio of CSS pixels to device-independent pixels is the scale factor or zoom level of the web page.

如果網(wǎng)頁未指定視口，則移動瀏覽器將以800至1024 CSS像素的后備寬度呈現(xiàn)該網(wǎng)頁。系統(tǒng)會調(diào)整網(wǎng)頁的比例系數(shù)，以便網(wǎng)頁與顯示屏大小相協(xié)調(diào)，這就要求用戶在與網(wǎng)頁互動之前先進行縮放。

元視口代碼會為瀏覽器提供指示，說明如何控制網(wǎng)頁的尺寸和比例，且文檔標頭中應包含此代碼。

The viewport can be set to a specific width, such aswidth=320或width=1024。雖然我們不建議這樣做，但這能有效確保具有固定尺寸的網(wǎng)頁能按預期顯示。

Use meta-viewport valueswidth=device-width可指示網(wǎng)頁與屏幕寬度（以設(shè)備無關(guān)像素為單位）進行匹配。這樣，網(wǎng)頁就可以重排內(nèi)容以匹配不同的屏幕尺寸。

有些瀏覽器（包括iOS和Windows Phone）在旋轉(zhuǎn)為橫向模式時會保持網(wǎng)頁寬度不變，并且會縮放（而非重排）網(wǎng)頁以填充屏幕。添加屬性initial-scale=1均可指示瀏覽器將CSS像素與設(shè)備無關(guān)像素的比例設(shè)為1:1（而不用考慮設(shè)備的屏幕方向），并且可讓網(wǎng)頁利用整個橫向?qū)挾取?/p>

Web pages must adapt to different widths to use the response viewport. For suggestions, please refer to our調(diào)整內(nèi)容的尺寸，使其符合視口設(shè)置的建議。

您可以設(shè)置縮放級別的下限和上限，也可以徹底禁止用戶縮放視口。這些選項會對可訪問性造成負面影響，因此一般情況下您應避免使用這些選項。

Although meta-viewport code is widely supported, it is not yet a formal standard. This action will act as aCSS Device Adaptation規(guī)范的一部分納入CSS。本規(guī)范最終定稿并廣泛實施之前，作者應繼續(xù)使用元視口代碼，從而確保兼容性。您可以單獨使用，也可以與相應的@viewport樣式搭配使用。

• In the Safari Web Content GuideConfiguring the Viewport一文。
• 怪異模式“雙視口記”：第一部分和第IIPart.
• Published in A List PartA Pixel Identity Crisis（像素定義引發(fā)的認同危機）一文。
• OperaAndMozillaThe viewport document provided on.
• Published in HTML5 RocksCreating a Mobile-First Responsive Web Design（創(chuàng)建移動先行響應網(wǎng)頁設(shè)計）一文。

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