After skimming through the Outlook website, I couldn't help but feel nauseated when peeking at the source code Outlook.com:
Their CSS leaves much to be desired... take a look for yourself.
In my opinion, creating a dual-column page with set pixel widths (absolute values) seems like a feasible approach. By assigning a class of column width in pixels to two separate sections—one displaying an image and the other showcasing login details/everything else—you can ensure that if the page is resized, they will stack vertically, ensuring smooth degradation.
You could potentially align an image and a login box without relying on CSS by configuring the login box to be centered with the image. However, this workaround is inefficient and involves using outdated code.