:: after for inline html?

Question

.banner  {
  width: 34px;
  height: 52px;
  position: relative;
  color: white;
  font-size: 11px;
  letter-spacing: 0.2em;
  text-align: center;
  float: right;
  padding-left: 10px;
  padding-top: 4px;
}

.banner:after { # How can I make this same behavior work inline?
  content: "";
  position: absolute;
  left: 0;
  bottom: 0;
  width: 0;
  height: 0;
  border-top: 24px solid transparent;
  border-bottom: 25px solid transparent;
  border-left: 9px solid white;
}

This is what produces:

How to create this inline form?

All that I tried does not work, because it seems that positionnegative fields are not supported in email too.