The number one reason for framing something is to preserve it, making it look nice is just a by-product of the framing process. That means that it is my job to do the artwork no harm. You can buy frames from the likes of IKEA that look perfectly serviceable and have perfectly cut mounts made with machines that have the tiniest tolerances HOWEVER the reason people still use real picture framers is to help preserve the precious artwork, needlework or photographs and the biggest threat to the subject is the frame itself particularly acid in the bleach for the card in the mount and backing board.
Conservation is one of the most used words in framing and in order to conserve the artwork you need conservation quality materials. everything I use is (unless specifically requested) is compliant with the Fine Art Guild conservation standard and I use the framing methods that they advise in their study guide. If you would like to know more I have a lot of literature about chloride levels, the dangers of lignin, and photograph activity tests!
I was cleaning out the harvest flies for a customer in a painting that was not especially old to discover this discolouration on the edges of the artwork. This just goes to show how important using good quality ACID-FREE materials are in framing. Also, the method of attaching the painting to keep it in place is not correct and could lead to the paper buckling with age.