According to http://www.w3schools.com/dom/prop_element_previoussibling.asp
The previousSibling property returns the previous sibling node (the previous node in the same tree level) of the selected element
If there is no such node, this property returns null.
technically, what you're showing there is
...so if the browsers are following the rules, it should keep working properly. If you're asking whether or not there are browsers out there that fail to follow the rules on this, I can't help you, but I will note that IE has a habit of adding dom objects to pages (particularly as wrappers) which might prove hazardous regardless.
Edit: http://www.w3schools.com/dom/dom_nodes_navigate.asp has this to say on the topic.
Firefox, and some other browsers, will treat empty white-spaces or new lines as text nodes, Internet Explorer will not.