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styling should not appear in any HTML
output. This ban includes all style attributes and inline <style>..</style> blocks.
I strongly suggest looking at PHPTAL over any other templating engine I've come across. This will assist us with security (automatically escaping strings to prevent XSS attacks), internationalisation and ease of template maintenance. The templates are valid XML documents and can be rendered directly in a web browser without needing to be parsed through the templating engine.
Come to some sort of arrangement on how plugins will manipulate the HTML
to insert their own buttons, widgets, etc as required.
Look at supporting right-to-left (RLT) in the redesign for languages that require this functionality.
Investigate accessibility concerns in depth to determine how the pages will work in screen readers, on browsers with extra large font sizes, for colour blind users, etc.
Avoid dumbing down the interface to cater for computer illiterate users (and making the interface look pretty) at the expense of making life hard for “power users”. Power users are those who are attached to MantisBT for many hours of the day triaging issues, scheduling work, creating reports, etc and they desire the most efficient workflow possible. Having to find functionality that has been hidden away behind multiple mouse clicks is going to frustrate them.