Contrast
Dropbox
Contrast is the practice of making design elements look different from each other in order to make the important elements stand out, it also can be used to create a focal point and establish visual interest by manipulating differences in color, size, space and other factors.
Alignment
Amazon
Alignment as an effective web design principle allows us to make conscious decisions about how and where elements are placed on the page.
Repetition
Versace
Repetition in web design is to simply format your hyperlinks in the same way on every single page, so users know what to expect when they click on a link.