On classes, identifiers, and descendant selectors
If you have the misfortune to work on a huge, poorly developed site where you will never have permission to refactor the templates and CSS according to common sense and best practices, you may have to rely on class names and avoid descendant selectors and ids.
But under almost any other circumstance, properly used ids with descendant selectors are preferable because more semantic and lighter in bandwidth.
As for descendant selectors, in a site not designed by 100 monkeys, it is safe to assume that elements within an id’d div or HTML5 element will be visually styled in ways that are compatible, and that those same elements may be styled differently within a differently id’d div or HTML5 element.
There is nothing wrong with id when it is used appropriately (semantically, structurally, sparingly). There is plenty wrong with the notion that class is always preferable to descendant selectors and semantic, structural ids.L. Jeffrey Zeldman