Come creare un triangolo rettangolo, sfumatura

Utilizzare un conic-gradient

.play {
  -webkit-appearance: none;
  appearance: none;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  width: 90px;
  height: 90px;
  cursor: pointer;
  background: conic-gradient(from -135deg at right,red 90deg,#0000 0) 55% 50%/22px 44px no-repeat;
  border: 9px solid red;
  border-radius: 50%;
  }

<button class="play" type="button" aria-label="Close"></button>

HTML

<button class="play" type="button" aria-label="Close">
  <div id="triangle-right"></div>
</button>

CSS

.play {
  -webkit-appearance: none;
  appearance: none;
  box-sizing: border-box;
  margin: 0;
  padding: 0;
  width: 90px;
  height: 90px;
  cursor: pointer;
  /*background:
    linear-gradient(green 0 0),
    linear-gradient(green 0 0)
    blue;*/
  background-size:7px 100%,100% 7px;
  background-position:center;
  background-repeat:no-repeat;
  border: 9px solid red;
  border-radius: 50%;
  transform: rotate(0deg);
}
#triangle-right {
  width: 0;
  height: 0;
  border-top: 20px solid transparent;
  border-left: 20px solid red;
  border-bottom: 20px solid transparent;
  margin-left: 30px;
}

Qui sta lavorando demo https://jsfiddle.net/rgn8wsbj/