Is the inputmode attribute valid (in HTML5 forms) or not?

The HTML5 spec says

The following content attributes must not be specified and do not apply to the element: accept, alt, checked, dirname, formaction, formenctype, formmethod, formnovalidate, formtarget, height, inputmode, max, min, src, step, and width.

It's under bookkeeping details at https://html.spec.whatwg.org/multipage/input.html#e-mail-state-(type=email)

Five years after the question was asked, some may wonder why some of the properties listed by @dsas doesn't trigger such errors, like enctype The answer is simple support, while enctype for instance gained a wide support inputmethod is supported only as of IE11 and Edge 14, for more infos click here

Also, see the chart about which attributes apply to the different input types here: http://www.whatwg.org/specs/web-apps/current-work/multipage/the-input-element.html#attr-input-type

The "inputmode" attribute applies only to "text" and "search".

UPDATE 2019-09-04: "inputmode" is now a global attribute (per WHATWG) and can be specified on any HTML element: https://html.spec.whatwg.org/multipage/dom.html#global-attributes

Another reference page for "inputmode": https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/inputmode

On another note, "inputmode" is not a W3C HTML5 attribute, but it is a W3C HTML 5.1 attribute (at least at the time I'm writing this). UPDATE 2019-09-04: "inputmode" has been removed from HTML 5.2 and HTML 5.3.