<input type="image"> - HTML: Hypertext Markup Language
supported common attributes alt, src, width, height, formaction, formenctype, formmethod, formnovalidate, formtarget idl attributes none.
...dditional attributes in addition to the attributes shared by all <input> elements, image button inputs support the following attributes: attribute description alt alternate string to display when the image can't be shown formaction the url to which to submit the data formenctype the encoding method to use when submitting the form data formmethod the http method to use when submitting the form formnovalidate a boolean which, if present, indicates that the form shouldn't be validated before submission formtarget a string indicating a browsing context from where to load the results of submitting the form height the height, in css pixels, at which to draw the image src the url from which to ...
... formmethod a string indicating the http method to use when submitting the form's data; this value overrides any method attribute given on the owning form.
... formmethod html5 specifies the http method that the browser uses to submit the form.

HTMLButtonElement - Web APIs
htmlbuttonelement.formmethod is a domstring reflecting the http method that the browser uses to submit the form.
... the following attributes have been added: autofocus, formaction, formenctype, formmethod, formnovalidate, formtarget, labels, validity, validationmessage, and willvalidate.

HTMLInputElement - Web APIs
formmethod string: returns / sets the element's formmethod attribute, containing the http method that the browser uses to submit the form.
... the following properties have been added: autocomplete, autofocus, dirname, files, formaction, formenctype, formmethod, formnovalidate, formtarget, height, indeterminate, labels, list, max, min, multiple, pattern, placeholder, required, selectiondirection, selectionend, selectionstart, step, validationmessage, validity, valueasdate, valueasnumber, width, and willvalidate.

<input type="submit"> - HTML: Hypertext Markup Language
ubmit"> additional attributes in addition to the attributes shared by all <input> elements, submit button inputs support the following attributes: attribute description formaction the url to which to submit the form's data; overrides the form's action attribute, if any formenctype a string specifying the encoding type to use for the form data formmethod the http method (get or post) to use when submitting the form.
... formmethod a string indicating the http method to use when submitting the form's data; this value overrides any method attribute given on the owning form.

<input>: The Input (Form Input) element - HTML: Hypertext Markup Language
ch name of form field to use for sending the element's directionality in form submission disabled all whether the form control is disabled form all associates the control with a form element formaction image, submit url to use for form submission formenctype image, submit form data set encoding type to use for form submission formmethod image, submit http method to use for form submission formnovalidate image, submit bypass form control validation for form submission formtarget image, submit browsing context for form submission height image same as height attribute for <img>; vertical dimension list almost all value of the id attribute of the <datalist> of autocom...
... formmethod valid for the image and submit input types only.

HTML attribute reference - HTML: Hypertext Markup Language
formmethod <button>, <input> if the button/input is a submit button (type="submit"), this attribute sets the submission method to use during form submission (get, post, etc.).

<button>: The Button element - HTML: Hypertext Markup Language
formmethod html5 if the button is a submit button (it's inside/associated with a <form> and doesn't have type="button"), this attribute specifies the http method used to submit the form.

<form> - HTML: Hypertext Markup Language
this value is overridden by formmethod attributes on <button>, <input type="submit">, or <input type="image"> elements.

<input type="range"> - HTML: Hypertext Markup Language
note: the following input attributes do not apply to the input range: accept, alt, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, maxlength, minlength, multiple, pattern, placeholder, readonly, required, size, src, and width.