don't confuse Chapter Breaks whit Header Margins (Headers being what most chapter titles/numbers use)
You may want to adjust : margin-
top and margin-
bottom
Your document is really
a stack of boxes.
H# boxes on top of P,Div or Blockquote boxes and kept in place by the Body Box which is wrapped (and labeled) with the HTML stuff.
Read up (and understand) on the HTML 'Box Model'.
Then you will know how to use and avoid misuse of empty lines and
BR 
Margin (IMHO the most common used)
Padding (for those special cases to keep things away from the
Border)
Text-Indent
All the above can be used to affect the edges of the Text or Image and without looking at the stylesheet can be hard to distinguish which was used to obtain what you see.
There are LOTS more to stylesheets that affect the Look and arrangement of the text but a KISS stylesheet works very well for simple documents.
BTW Continuous Chapter flow (chapter ending and next chapter start on the same page) went out in the 60's
