@hint/hint-meta-viewport

hint for best practices related to the meta viewport

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Correct viewport (meta-viewport)
meta-viewport warns against not having a single viewport meta tag in the <head> with the proper value.

Why is this important?

The viewport meta tag is an essential part of responsive web design, that may also offer some performance improvementsgpu rasterization.
Mobile browsers render pages in a virtual "window" (the viewport), usually wider than the screen, so they don’t need to squeeze every page layout into a tiny window (which would break many non-mobile-optimized sites). Users can pan and zoom to see different areas of the page.
Mobile Safari introduced the "viewport meta tag" to let web developers control the viewport’s size and scale. Many other mobile browsers now support this tag.
In recent years, screen resolutions have risen to the size that individual pixels are hard to distinguish with the human eye. For example, recent smartphones generally have a 5-inch screens with resolutions upwards of 1920—1080 pixels (~400 dpi). Because of this, many browsers can display their pages in a smaller physical size by translating multiple hardware pixels for each CSS "pixel". Initially this caused usability and readability problems on many touch-optimized web sites.

Using the viewport meta tag to control layout on mobile devices (MDN)viewport meta tag on mdn
The viewport related topic is very complex so if you want to dig deeper, read Peter-Paul Koch’s "A tale of two viewports" part oneppk article 1 and part twoppk article 2, or watch his talk 'The Mobile Viewports'ppk talk.
NOTE: If your website is not responsive, then this meta tag might not be needed.
Ideally the following meta viewport tag should be used:
<meta name="viewport" content="width=device-width, initial-scale=1">

Or, if most of your users don’t use Safari for iOS < 9:
<meta name="viewport" content="width=device-width">

Notes:
  • It is recommended to use:

width=device-width
* `device-width` will make the page match the screen’s width in
  device-independent pixels, allowing its content to reflow to
  match different screen sizes.

  Setting the `width` property to a specific size (e.g.: `width=320`)
  is [not recommended][fixed width problem].

* Having `width=device-width` also constitutes a performance
  improvement, as under most circumstances, it enables fast tapping,
  removing the 300-350 ms tap delay on [Safari for iOS 10+][ios 10
  interaction behaviors] and [other mobile browsers][tap delay].
 initial-scale=1
* This is mostly needed to [work around the orientation change bug
  from Safari for iOS < 9][ios orientation change scaling].

* Using values different then `1` (or `1.0`) are
  [problematic](https://www.quirksmode.org/mobile/metaviewport/#link15).
  • user-scalable, maximum-scale, and minimum-scale properties
should not be used.
These properties can block the user from zooming on a page. With such a wide range of devices available with different display dimensions, screen resolutions, pixel densities, etc., it is difficult to choose an appropriate text size in a design. Most of the time using these properties enable users to pick a text size that is unreadable while preventing them from zooming, frustrating them, or making the web site/app inaccessible in some cases.
Considering the issues described, these properties are now ignored by some mobile browsers such as Safari for iOS 10+ios 10 interaction behaviors.

What does the hint check?

The hint checks if the viewport meta tag was specified a single time in the <head>, and if:
  • the width property is provided and its value is device-width
  • the initial-scale property is provided (note: depends on the
configurations) and its value is 1 or 1.0
  • user-scalable, maximum-scale, or minimum-scale are used
  • it includes unknown properties (e.g.: x=y) or invalid values
(width=x)

Examples that trigger the hint

The viewport meta tag is not specified in <head>:
<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>example</title>
        ...
    </head>
    <body>
        <meta name="viewport" content="width=device-width, initial-scale=1">
    </body>
</html>

The viewport meta tag contains an unknown property:
<meta name="viewport" content="unknown-property=1, width=device-width, initial-scale=1">

The viewport meta tag contains an invalid value:
<meta name="viewport" content="width=invalid-value, initial-scale=1">

The viewport meta tag contains a disallowed property (user-scalable):
<meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no">

The viewport meta tag contains a fixed width value:
<meta name="viewport" content="width=320, initial-scale=1">

The viewport meta tag contains initial-scale with a value different than 1 or 1.0:
<meta name="viewport" content="width=device-width, initial-scale=5">

There are multiple viewport meta tags:
<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>example</title>
        <meta name="viewport" content="width=device-width, initial-scale=1">
        ...
        <meta name="viewport" content="width=device-width">
        ...
    </head>
    <body>...</body>
</html>

Examples that pass the hint

If versions of Safari for iOS < 9 are targeted:
<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>example</title>
        <meta name="viewport" content="width=device-width, initial-scale=1">
        ...
    </head>
    <body>...</body>
</html>

<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>example</title>
        <meta name="viewport" content="width=device-width, initial-scale=1.0, shrink-to-fit=no, viewport-fit=cover">
        ...
    </head>
    <body>...</body>
</html>

If versions of Safari for iOS 9+ are targeted:
<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8">
        <title>example</title>
        <meta name="viewport" content="width=device-width">
        ...
    </head>
    <body>...</body>
</html>

Can the hint be configured?

This hint takes into consideration the targeted browsersbrowser configuration, and if no versions of Safari for iOS < 9 are included, it will not require initial-scale=1.

How to use this hint?

This package is installed automatically by webhint:
npm install hint --save-dev

To use it, activate it via the .hintrchintrc configuration file:
{
    "connector": {...},
    "formatters": [...],
    "hints": {
        "meta-viewport": "error",
        ...
    },
    "parsers": [...],
    ...
}

Note: The recommended way of running webhint is as a devDependency of your project.

Further Reading