Less mixin to emulate :focus pseudo-class for IE7. Probably works under IE 5-6 (not tested). Based on one-time expression (executed only once, doesn't affect performance)

focus-mixin.less

/* mixin */
.focus(@className: "focused") {
  behavior: ~"expression(function focus(el) { el.runtimeStyle.behavior = 'none'; var focusClass = ' @{className}'; el.onfocus = function() { el.className += (focusClass); }; el.onblur = function() { el.className = el.className.replace(new RegExp(focusClass, 'g'), ''); }; }(this))";
  }

/* usage */
.input {
  .focus("input_focused");

  .input:focus,
  .input_focused {
    border-color: blue;
  }
}