Is it okay to tag the form without sending anything to the server?

From the W3c Documentation https://www.w3.org/TR/html5/forms.html#introduction-1
Or
https://www.w3.org/TR/html51/sec-forms.html#forms-introduction

No client-side scripting is needed in many cases, through an API is available so that scripts can be authenticated the user experience or use forms for purpos other than submitting data to a server.

In most cases, it is not necessary, but the form is made available for other purposes besides sending it to the server (free translation).

In other words, if you want to use the form without sending it to the server,
"I don't recommend it positively, but I think it's okay to use it."

If you return false; in the onsubmit event, you can stop sending.

For example, simply
onsubmit="return false" should be added to the form tag as follows:

<form id="form_id" method="action="onsubmit="return false">
various forms
</form>

Even if you use a button that makes you submit,
You can return false; in the onsubmit event handler function.