Add Full Width Image Support in a WordPress Theme - CSS

full-width-image-theme-support.css

.alignwide {
  /* Set these margins to work with your own theme. */
  margin-left: -80px;
  margin-right: -80px;
  max-width: 100vw;
}

.alignfull {
  margin-left: calc(-100vw / 2 + 100% / 2);
  margin-right: calc(-100vw / 2 + 100% / 2);
  max-width: 100vw;
}

.alignfull img {
  width: 100vw;
}