Well for starters, this one looks dope. The logic and DOM magic is all coded in vanilla JavaScript and it's styled using basic CSS. I based the functionality on my iPhone's calculator because that's what I had on hand to test it all out.
Keep hitting equal after an equation and the calculator will continue evaluating based on the previous calculation. The percent button took me the longest to hack out. The iPhone calculator treats the percent button differently whether it's the first number in the equation or the second; also addition/subtraction vs multiplication/division. I wanted my calculator to reflect that, so for example you'll see that if you type 3 and then % the display will show 0.03 which is 3/100. However if you want to increase 3 by 6%, you can type 3 + 6% and the display will immediately show 6% of 3 before adding it all together by hitting the = key. Just TRY IT! Use your keyboard!
You can read more on the github repo.