Seems that not treat well comments in css between /* ... */

[yourchoice]

Invalid property name '/* 0.001, because opacity:0 doesn't trigger Paint action, which causes lag at start of transition /

Example photoswipe.css

/* style is added when JS option showHideOpacity is set to true */
.pswp--animate_opacity {
  /* 0.001, because opacity:0 doesn't trigger Paint action, which causes lag at start of transition */
  opacity: 0.001;
  will-change: opacity;
  /* for open/close transition */
  -webkit-transition: opacity 333ms cubic-bezier(0.4, 0, 0.22, 1);
          transition: opacity 333ms cubic-bezier(0.4, 0, 0.22, 1); }