Novalidate is a boolean attribute that indicates that the form shouldn't be validated when submitted. It is useful if you want the user to save the progress of form filing. If the form validation is disabled, the user can easily save the form and continue and submit later.
The method attribute is the HTTP method to submit the form with. The only allowed methods/values are post (form data is sent as the request body), get (form data appended to the action URL with a ? separator, used when form has no side effects), and dialogue (closes the dialogue and throws a submit event on submission without submitting data or clearing the form).
The action attribute is the URL that processes the form submission. The value can be overridden by a formaction attributes, and is ignored when method="dialog" is set.
<fieldset> is a HTML element that provides a gropuing for a part of an HTML form, while the <legend> element is nested inside of it, providing a caption for the fieldset.
The for attribute is an allowed attribute for <label> and <output>. When used as an attribute of <label>, the for attribute has a value which is the id of the form element it relates to. When used as an attribute of <output>, the for attribute has a value which is a space separated list of the id values of the elments which are used to create the output. For attributes must have an id attribute to pair with.
Overall, there are several different HTML elements, attributes, and values that go into creating the structure of forms. The ones listed in this documentation only describe a few that are regularly used. The attributes novalidate, method, and action indicate what forms should do when they're submitted. The <fieldset> and <legend> attributes provide grouping and captions for HTML forms. While the for and id attributes pair the <label> and <output> elements with the form element it relates to.