Div tree for subtitle

[ayu7377]

Using subtitle_event_data with one render_div, call to imsc.renderHTML(subtitle_event_data, render_div) makes these kind of div tree.

Through the codeflow of imsc_library, what I understood is it is creating structure under given render_div is like {global, region, body, div, p, span}.
In this tree, parent div having position: relative, and its child div having position: absolute is not inheriting and influencing p and span tag, because there are two div with no use (it seems no use, correct me if I'm wrong).
And as it is not inheriting, in one webpage, I faced the issue, my render_div was there but not visible on the screen. If I manually removed these two div "div style="margin: 0px; visibility: visible;", then subtitle were visible.

Can someone please explain what is the use of these div style="margin: 0px; visibility: visible;", what is the meaning of this strcyre {global, region, body, div, p, span}, and is it okay if I manually do not append these div style="margin: 0px; visibility: visible;".