Modal Windows for User Login: NOT best practice?

We are considering redesigning our client sign in page as a modal window but I am not clear if this is entirely against best practice. I like the design of modal windows. What are the challenges?

What are the reasons for using a modal window? What is it doing for you or your users?

Is this a home page with normal home page content where a user has access to a button or link called “sign in” and instead of going to a new page, gets a lightbox-like login?

If it’s something like that, there are generally some issues with general modals and AT sometimes, but if this is really just an in-page link to a form who is actually already on the page, just offscreen or something, and you’re just styling stuff to look lightboxy with CSS, then actually it shouldn’t be a huge issue there (it won’t be a true modal, which traps the user in the modal until they either explicitly hit ESC, Cancel, Ok or sometimes the Back button). Your CSS will have to die off whenever the user’s focus is not in the “modal” area then. This is common even with Javascripted lightboxes: try tabbing with TAB and you’ll find you’re tabbing through the mostly-hidden page underneath now. Not good.

I suppose one possible benefit of a modal is, unlike a real link taking users to a dedicated login page, we are saving an HTTP request. Like a dedicated page, the user is presented the form without other page clutter in the way. No distractions. However it seems most pages who need logins get logged into without using either separate dedicated pages or modals.

I won’t say more until you describe what you’ve got and why a bit more. Cheers.