theming – Escaping CSS digits, 0 is removed from CSS aggregated files

The following HTML markup won’t work on a production site.

<div class="iconleft 109">TEXT</div>

This is the code in the custom.css file.

.iconleft.31 09,
.iconleft.31 12,
.iconleft.31 10,
.iconleft.31 11,
.iconleft.31 20 {
  background: #B0CD00;
}

.iconleft.31 09 works on localhost with xampp and no aggregation. On the production site, where Advanced CSS/JS Aggregation is activated, it wont work for 109.

In the aggregated CSS file, I see .iconleft.31 9 as CSS class. It seems that 0 has been removed.

Do you have any hint?