I have read in forums and in some old documentation, that there is a default margin on the body tag of 8px. But on This Site I read that all the default styling attributes of body tag has been deprecated in html5.
But I found this default margin appearing in Firefox during an html5 practice session. Is it some specific issue with the browser, or it isn't deprecated?
Browsers have supported topmargin, leftmargin, etc as attributes you could add to the <body> start tag to control margins.
These have never been part of any HTML specification. (If they had been, then they would have been deprecated in HTML 4). They are non-standard extensions to HTML introduced by browsers during the late '90s.
HTML 5 marks then as non-conforming features (which is not the same as deprecated, but the document you found conflates the two states).
The default stylesheet that browsers apply to HTML documents includes a margin for the body element. This is not deprecated (or mandated by any specification).