CSS using firebug with tab computed style question

I am trying开发者_开发百科 to use the firebug for looking at a layout in a select box. On the right column of the firebug if I select option 1 from the select table, I can see in the computed column,

font-family verdana,arial,helvetica,sans-serif font-size 13px font-weight 400

However, when I change the selection to the 2nd or 3rd selection, suddenly the whole page looses its layout, and in the computed column of firebug, there are some changes with these formats :

font-family serif font-size 16px font-weight 400

The strange thing is that I already delete all the css file related to this layout, but I can not udnerstand where it gets these different font everytime I change the selection of the select table. Any suggestion?

Thank you for any kindness response

Hmm, did you use CSS reset? Could it be that you're running into browser defaults?