CSS
docomo
Old docomo (Browser 1.0 or pre summer 2009) does not support external stylesheets. There are many libraries that handle programatically inlining CSS.
au
- The input field font size is not what you would expect from the css, they are all defaulted to the default, largish size. To change, set explicitly in the input tag or create a class for au inputs.
UP.Browser/6.2
This browser's rendering of CSS is quite limited with regard to classes. Only one CSS class per element is supported. Also, a css declaration can only have one class in it.
SoftBank
SoftBank 822P. The default font color will be applied to input fields as well. So if you have white text, the input text will be white as well! To remedy this, add something like input {color: black;} to the page style.
media
Be sure to set the media type to be handheld,tty, i.e.,
<link rel="stylesheet" type="text/css" href="mobile.css" media="handheld, tty" />