Hi snehula, good on you for validating, and I think you are correct that the warnings about the properties that don't exist in css2.1 shouldn't be affecting the layout. However, the parse error could because it may be affecting the way subsequent rules are interpreted/applied - so I'd get that resolved.
To return to your original issues though, you've said you had your list elements set to 100% width - and you are right that should be 100% of the parent element which is the <ul>. However, if the list items are occupying the full width of the parent, they cannot float to the left of each other because there isn't enough horizontal room. ie copes with percentages quite well, and in this case is actually behaving correctly, so the fact that some browsers are floating the <li> next to each other suggests there is something else affecting this.
Given your comments about the css and problems with floating in earlier threads my first guess at the source of these conflicting displays would be invalid code - either html or css. So at this stage I'd suggest to keep validating until you have removed all parse errors. After that, if the problem still exists can you post an "examplified" html snippet that shows the mark-up for the list, plus the relevant relevant css.