This feature is mostly interesting for people writing in East Asian languages like Japanese, Korean or Chinese.
Text in these language are usually written using input mode editors. However, in a web form some fields might require text in a ideographic script, while other might require input using latin characters or numbers, which shouldn't activate the input mode editor.
inputmode attribute can be used to hint what script is expected. This helps the browser
decide whether the input mode editor should be activated, or standard keyboard input should be used.
The text is not required to be in the specified script, its just a hint to the user interface.
Note that the IME will only be activated if the keyboard configuration is a East Asian language, and the corresponding input method editior is installed.
This is the source:
<input type="text" inputmode="katakana">
Here is a form with different fields: