Accessibility requirements have got stricter in some jurisdictions requiring sites to comply with the W3 WCAG guidelines. Use of a third-party checking tool (in this case "Wave") shows a few things to change to comply with such requirements when targeted against a new IdP4 login page.
The WAVE Firefox plugin checking the login page shows 1x Error, 1x Alert and some things to consider (which probably don't apply to a login page anyway):
The language of the document is not identified.
Identifying the language of the page allows screen readers to read the content in the appropriate language. It also facilitates automatic translation of content.
Identify the document language using the <html lang> attribute (e.g., <html lang="en">).
The <html lang> attribute is missing or is empty.
The page has no headings.
Headings (<h1>-<h6>) provide important document structure, outlines, and navigation functionality to assistive technology users.
Provide a clear, consistent heading structure, generally one <h1> and sub-headings as appropriate. Except for very simple pages, most web pages should have a heading structure.
No <h1>-<h6> elements are present in the page.