In design, reflections are stylized mirror images of objects. Even though they are not as popular as shadows, they have their moments — just think about the first time you explored the different font formats in MS Word or PowerPoint: …
Clipping and masking have been around for a while now in CSS and even have pretty decent browser support. I recently worked on a project that needed to use a clipping technique for tooltips showing above links in text.…
mask property in CSS allows you to hide parts of an element. For example, if you have a black and white image, you can apply that as a mask and the black parts will force the element to be transparent on that elements. Masks can also match the alpha transparency of the mask image. 
Tyler Gaw reminds us that mask-image can repeat, resize, and move just like background-image does, which is fun to combine and play with. I could see it being a fun option for an <hr, like Sara is doing. …