Answer 1

box-sizing: border-box

Answer 2

大概实现了一下？返回值和原生略有不同使用了对象方便访问：

function myInterval(func, duration) {
  let tag = {
    flag: +new Date
  }
  const f = () => setTimeout(() => {
    if(tag.flag){
      func()
      f()
    }
  }, duration)
  f()
  return tag
}

function myClear(tag) {
  tag.flag = 0
}

let t = myInterval(() => console.log(1), 3000)

myClear(t)