Hmm... it's hard for me to read your code, but I'd venture to say it's cause you have your sub-content and main-content as the same width, or something to that extent. Sub content should be inside your main-content div.
Also from an execution standpoint you want to write your code to where it make sense... like you have main-content coming before nav-tier1, but in the page layout it comes after. so in your code you should have it coming after.
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