New: Add rule to check the usage of web app manifest's short_name & name

docs/user-guide/rules/index.md

@@ -26,6 +26,7 @@
 ## PWAs
 
 * [`apple-touch-icons`](apple-touch-icons.md)
+* [`manifest-app-name`](manifest-app-name.md)
 * [`manifest-exists`](manifest-exists.md)
 * [`manifest-file-extension`](manifest-file-extension.md)
 

docs/user-guide/rules/manifest-app-name.md

@@ -0,0 +1,119 @@
+# Require manifest to specify the web site/app name (`manifest-app-name`)
+
+`manifest-app-name` warns against not specifying the web site/app's
+name.
+
+## Why is this important?
+
+Browsers that have support for the [web app manifest file][manifest
+spec] will use [`name`][manifest name] member (or the
+[`short_name`][manifest short_name], when there is insufficient space)
+to display the name of the site/app in situations such as: amongst
+a list of other applications, as a label for an icon, etc.
+
+If these members are not defined, browsers will try to get the name
+from other sources such as the value of the [`application-name` meta
+tag, `<title>`, or just default to a specific value (e.g.:
+`Untitled`)][manifest metadata]. This can lead to bad user experience,
+as the web site/app name may be truncated or just wrong.
+
+In general it is recommended to specify and have the `name` member
+under 30 character, and if it's over 12 characters also have a
+`short_name` member that is at most 12 characters.
+
+Notes:
+
+* If the `name` is under or 12 characters, there isn't a need to
+  specify `short_name` as browsers can just use `name`.
+
+* The 12 character limit is used to ensure that for most cases the
+  value won't be truncated. However depending on [other things][sonar
+  issue], such as:
+
+  * what font the user is using
+  * what characters the web site/app name includes (e.g. `i` occupies
+    less space then `W`)
+
+  the text may still be truncated even if it's under 12 characters.
+
+* The 30 character limit is used in order to be consistent with the
+  native OSes/[app stores][app store] limits/recommendations.
+
+## What does the rule check?
+
+The rule checks if a non-empty `name` member was specified and it's
+value is under 30 characters.
+
+If the `name` member is over 12 characters, or `short_name` is
+specified, the rule will also check if `short_name` has a non-empty
+value that is under 12 characters.
+
+### Examples that **trigger** the rule
+
+Manifest is specified without `name` and `short_name`:
+
+```json
+{
+    ...
+}
+```
+
+Manifest is specified with a `name` longer than 12 characters
+and no `short_name`:
+
+```json
+{
+    "name": "Baldwin Museum of Science",
+    ...
+}
+```
+
+Manifest is specified with a `name` longer than 30 characters:
+
+```json
+{
+    "name": "Baldwin Museum of Science - visit today!",
+    "short_name": "Baldwin"
+    ...
+}
+```
+
+Manifest is specified with `short_name` longer than 12 characters:
+
+```json
+{
+    "name": "Baldwin Museum of Science",
+    "short_name": "Baldwin Museum"
+    ...
+}
+```
+
+### Examples that **pass** the rule
+
+Manifest is specified with a `name` shorter than 30 characters
+and a `short_name` shorter than 12 characters:
+
+```json
+{
+    "name": "Baldwin Museum of Science",
+    "short_name": "Baldwin"
+    ...
+}
+```
+
+Note: [Not specifying a manifest file](manifest-exists.md), or having
+an invalid one are covered by other rules, so those cases won't make
+this rule fail.
+
+## Further Reading
+
+* [Web App Manifest specification][manifest spec]
+
+<!-- Link labels: -->
+
+[app store]: https://developer.apple.com/app-store/product-page/
+[manifest metadata]: https://w3c.github.io/manifest/#authority-of-the-manifest%27s-metadata
+[manifest name]: https://w3c.github.io/manifest/#name-member
+[manifest short_name]: https://w3c.github.io/manifest/#short_name-member
+[manifest spec]: https://w3c.github.io/manifest/
+[sonar issue]: https://github.com/sonarwhal/sonar/issues/136