All of the 'normal' attributes get passed in as part of the object HtmlAttributes parameter. So you would do something like this..
<% object parms = new{ onclick="javascript:Login();", style="width:100px" } ; %>
<%= Html.Button("btnLogin", "Login", parms) %>
I forget the order of the Button() parameters. I'm pretty sure name is first then caption, then HtmlAttributes. You can always include the object code inline within the argument list for Button(), I just like to do it like I have it above.