<div class="container">
  <div class="box">
    <button type="submit">Click Me!</button>
  </div>
</div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;

  display: flex;
  justify-content: center;
  align-items: center;
  gap: 10px;

  font-family: "Exo", Arial, sans-serif;
  background-color: #220;
  color: #fff;
  transition: all 150ms linear;
}

button {
  display: inline-flex;
  justify-content: center;
  align-items: center;
  padding: 12px 20px;
  cursor: pointer;
  user-select: none;
  transition: all 150ms linear;
  color: #fff;
  border: 0 none;
  border-radius: 10rem;
  font-size: 1.5rem;
  font-weight: 500;
  appearance: none;
  flex: 0 0 200px;
  box-shadow: 0px 5px 2px #e4e4e4;
  background-color: #416dea;
}

button:hover {
  transition: all 150ms linear;
  opacity: 0.85;
}

button:focus:not(:focus-visible) {
  outline: 2px dotted #416dea;
  outline-offset: 2px;
  box-shadow: 0px 1px 1px #416dea;
}

button:focus-visible {
  outline: 2px solid #416dea;
  outline-offset: 2px;
  box-shadow: 0px 1px 1px #416dea;
}

.container {
  width: 50vw;
  height: 50vh;
  display: flex;
  justify-content: center;
  align-items: center;

  border: 1px solid #f09;
  border-radius: 5px;
  padding: 40px;
}

.box {
  width: 100%;
  height: 100%;
  border: 1px dashed #09f;

  display: flex;
  justify-content: center;
  align-items: center;
  padding: 30px;
  border-radius: 5px;
}

div {
  position: relative;
}

div::before {
  content: attr(class);
  position: absolute;
  white-space: nowrap;
  left: 2px;
  bottom: calc(100% + 2px);
  background-color: rgba(0, 0, 0, 0.7);
  padding: 5px 10px;
}

.box:focus-within,
.container:focus-within {
  box-shadow: 0 0 5px 3px rgba(255, 255, 255, 0.65);
}

body:focus-within {
  background-color: #2a0f5bde;
}

html:focus-within {
  border: 5px solid #09f;
}