Keyword Analysis of HTML5
Using the Google External Keyword Tool, it appears that “HTML 5” (with the space) and “HTML5” (without the space) returned all the exact same numbers for “Competition”, “Global Monthly Searches”, “Local Monthly Searches”, and “Local Search Trends”.
Google must be treating the two as the same search. So it wouldn’t matter if you write your articles with “HTML5” or “HTML 5”.