Google wants snippets to accurately represent the web results.They frequently prefer to display meta descriptions of pages (when available) because it gives users a clear idea of the URL's content. This directs them to good results faster and reduces the click-and-backtrack behavior that frustrates visitors and inflates web traffic metrics. Keep in mind that meta descriptions comprised of long strings of keywords don't achieve this goal and are less likely to be displayed in place of a regular, non-meta description, snippet. It's worth noting that while accurate meta descriptions can improve clickthrough, they won't affect your ranking within search results.
Google can read both HTML and XHTML-style meta tags, regardless of the code used on the page