How to compose css?

Question

css

Tell a beginner how to make such a frame for text without a plugin?

Answer 1

That's about what you need https://codepen.io/anon/pen/YemNLX
There, play around with styles to make it look more like

Answer 2

Below is a pseudo-element with a shadow, on top of it is a semi-circle gradient.

.foo {
  position: relative;
  border: solid 1px gray;

  width: 50%;
  padding: .5em;
  background: white;
}
.foo:after {
  content: '';
  position: absolute;
  left: 2%; right: 2%; bottom: -21px;
  height: 20px;
  background: radial-gradient(circle at bottom, white 65%,transparent 100%);
  z-index: -1;
}
.foo:before {
  content: '';
  position: absolute;
  left: 2%; right: 2%; bottom: 0px;
  height: 10px;
  box-shadow: 0px 0px 25px 5px rgba(0,0,0,0.85);
  z-index: -2;
}

https://jsfiddle.net/vgj0mp99/
too lazy to pick up the exact numbers.