.obrigatorio {
    /* animacao */
    animation: obg .2s;
    animation-iteration-count: 4;
    border-bottom: 1px solid #ff0000;
  }
  
  /* keyframe para ajustar até onde treme */
  @keyframes obg {
    0% {margin-left: 0;}
    25% {margin-left: 7px;}
    50% {margin-left: 0;}
    75% {margin-left: -7px;}
    100% {margin-left: 0;} 
  }