execCommand behaviours

Hi all,

In which part of the mozilla source code can i find the behaviour of
browsers execCommands (ctrl+b, ctrl+i, etc) when they are invoked in
'contentEditable = true' elements.

Basically i wanted to know all the default behaviours of browser's
execCommands and override them in **javascript** so that any exec
command creates only 'SPAN' and 'P' tags. for example when i select a
content and press 'ctrl+b' instead of <b> tag i need to put <span>
with appropriate styles.

Please help.

Thanks in advance,
Saravanan.S