A major consideration when creating new page layouts in SharePoint Designer is how you style those pages, such as using tables or DIV/CSS layout. The out-of-the-box page layouts are mostly designed to comply with XHTML 1.0 strict specification as defined by the Worldwide Web Consortium (W3C). Some page layouts, such as the EnterpriseWiki.aspx page layout, use fluid, or CSS, layouts.
Default page layout styles are located in the site collection’s Style Library:
The following style sheet files are referenced within several page layouts:
• Edit-mode-21.css: Styles that relate to a page when in edit mode.
• Page-layouts-21.css: Styles ...