Drop-down CSS Menu Breaks in Internet Explorer when the Page has DOCTYPE

To make the Dropdown CSS menu with DOCTYPE work in Internet Explorer browser, perform the following steps:

1. Edit style of your Page tabs i.e. div id='outerwrapper' by navigating to Setup> Site Builder > Themes. 
2. Edit the current theme.
3. In Addition to head section, add this code:

                #outerwrapper {
                    width: 800px ;
                    margin-left: auto ;
                    margin-right: auto ;
                }