Most webmasters and SEOs know by now that Google tends to typically use the title tag of the page as the blue, clickable link in the search results. But that may change based on query or other things. But what if you are missing a title tag in your page source, what would Google use?
I spotted a thread in the Google Custom Search Help forum discussing this. I should note, Google Custom Search is not the same as Google web search, but they do pass over similar algorithms and techniques, as one would imagine.
Googler, Prathap Reddy said in that thread:
Google uses the title of the document from file properties as title of the search results. If no title is found, the first found large font is used as title for the document, with some exceptions.
Now, again, I am sure this can be easily verified with some SEO testing but as far as Google Custom Search goes, if the title tag is not there, Google may use the first large font found on the page as the title (clickable link) from the search results.
Forum discussion at Google Custom Search Help.