The tag of<o:p> and the <?xml:namespace prefix = o /> in the html

That is microsoft office tags. You get it when you copy from office programs like word or outlook emails directly into your
web editor, without removing the microsoft formatted tags.And if you use the function of "Save as html" of MS word ,in the file of the target html file you can also find this tag.

it just for style and if you don't like this tag ,you can delete them .

Never copy from word or email and paste stuff directly into web. First, remove the format as follow:

Copy the text you want to use and paste it in Notepad. Then copy the clean, unformatted text from notepad into your website. Notepad is a text only editor, and it will strip any formatted text clean.