Okay I’m re-reading your original post, and it seems that the code you posted is the actual code that you have added in the HTML block. Please correct me if I have got this wrong.
If so, this is incorrect HTML. You are adding extra <html>, <head>, and <body> tags, which is the wrong way to do this. There should only be one of each of these on a web page, and these tags are already part of the rendered web page’s HTML markup. So, additionally adding these within the page will indeed “affect all other areas” as you say, and result in incorrect HTML markup.