In Safari, the «Back» command maps to ⌘-[, and «Forward» maps to ⌘-]. These are important commands, so it seems like it would make sense to map them to easily accessible keys, and the [ and ] keys certainly qualify.
On US keyboard layouts.
On German and Swiss German layouts, these keys do not exist at all. To invoke these commands on a German keyboard, you have to type ⌘-⌥-51 and ⌘-⌥-6, respectively, because ⌥-5 evaluates to «[» and ⌥-6 evaluates to «]».
In other words, for German and Swiss-German users, it’s virtually impossible to remember the command keys for two of the most important commands in Safari.
Sven-S. Porst wrote a post on keyboard commands. The important takeaway here is to avoid using non-alphanumeric characters for keyboard commands whenever possible; non-alphanumeric characters you take for granted may not even appear on keyboards different from yours. And since a lot of people will use your application using its US English interface while on a different keyboard layout, it’s probably a good idea to avoid such commands even in the US English version of your application.
There are some non-alphanumeric characters which can be used for keyboard commands since they appear on most keyboards, like the arrow keys. Use them wisely, and keep in mind that they often already have meaning when the focus is on a text field.
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