If you have low vision and use a screen reader or high contrast settings, you can configure the Office 365 Web App to use the Light version.
The light version of the Office 365 web interface provides fewer features and is generally recommended for those who need optimization for accessibility. A list of features can be found here: http://blogs.technet.com/b/ilvancri/archive/2010/04/07/exchange-2010-owa-versus-owa-light.aspx .
To use Office 365 light, you will need configure it. This is a one time step.