Proximity - when things are similar , group them together
Alignment - keep margins clean (justified text is actually not the best way to read) unify and organize the page
Repetition - of visual elements - using a theme that repeats throughout he website - Ex. a particular font. it is critical in a multipage document.
Contrast - pick a title font (serif) and a different one for your text body (sans-serif) - it frwas out eyes into it. they cannot be similar