fieldset.css

/**
 * @file
 * Fieldsets styling.
 *
 * Fieldset legends are displayed like containers in Seven. However, several
 * browsers do not support styling of LEGEND elements. To achieve the desired
 * styling:
 * - All fieldsets use 'position: relative'.
 * - All legend labels are wrapped in a single span.fieldset-legend that uses
 *   'position: absolute', which means that the LEGEND element itself is not
 *   rendered by browsers.
 * - Due to using 'position: absolute', collapsed fieldsets do not have a
 *   height; the fieldset requires a 'padding-top' to make the absolute
 *   positioned .fieldset-legend appear as though it would have a height.
 * - Various browsers are positioning the legend differently if there is a
 *   'padding-left'/'padding-right' applied on a fieldset and inherit the
 *   positioning even to absolute positioned elements within; we therefore have
 *   to apply all padding to the inner .fieldset-wrapper instead.
 */

fieldset .fieldset-legend {
  position: absolute;
  left: 0; /* LTR */
  top: 0;
  width: 100%;
  padding: 0.5em 0.78947em; /* LTR */
  text-transform: uppercase;
}
[dir="rtl"] fieldset .fieldset-legend {
  right: 0;
  padding-right: 0.9375em;
  padding-left: 0;
}

.fieldset-description {
  margin: 0 0 0.75em;
  padding: 0.75em 0.1875em;
}

.collapsible > .fieldset-wrapper > .fieldset-description {
  margin: 0 0 0.75em;
  padding: 0.75em 1.5625em;
}

/**
 * Collapsible Fieldsets
 */
.js fieldset.collapsible {
  position: relative;
  backface-visibility: hidden;
}

fieldset .fieldset-wrapper {
  padding: 0 13px 13px 15px; /* LTR */
}
[dir="rtl"] fieldset .fieldset-wrapper {
  padding: 0 15px 13px 13px;
}

.text-format-wrapper fieldset.collapsible .fieldset-legend {
  text-transform: none;
}

.js fieldset.collapsible .fieldset-title {
  position: relative;
  z-index: 1;
  display: block;
  width: 100%;
  padding: 0.3em 0.3em 0.3em 1.8em; /* LTR */
  margin: 0 -0.5em;
  border-radius: 4px;
  background: transparent;
  text-decoration: none;
}
.js[dir="rtl"] fieldset.collapsible .fieldset-title {
  padding: 0.3em 1.8em 0.3em 0.3em;
}

.js fieldset.collapsible .fieldset-legend a:before {
  content: "";
  position: absolute;
  left: 0.6em; /* LTR */
  top: 0.8em;
  width: 0;
  height: 0;
  border: .32em solid transparent;
  border-top-color: #000000;
  border-bottom: 0;
  transform: translate(0, 0.125em);
  transition: transform 0.25s ease-out;
}
.js[dir="rtl"] fieldset.collapsible .fieldset-legend a:before {
  right: 0.6em;
  left: auto;
  transform: translate(0, 0.125em);
}

.fieldset-legend span.summary {
  color: #757575;
  position: absolute;
  right: 1.4em; /* LTR */
  top: .8em;
  z-index: 0;
  /* Hide the summary by default; we show it on wider screens (see below). */
  display: none;
}
[dir="rtl"] .fieldset-legend span.summary {
  right: auto;
  left: 1.4em;
}
.fieldset-legend span.summary:not(:empty):before {
  content: '(';
}
.fieldset-legend span.summary:not(:empty):after {
  content: ')';
}
@media (min-width: 34em) {
  .fieldset-legend span.summary {
    /* Show the summary on wider screens. */
    display: block;
  }
}

/* Collapsed state styles */
.js fieldset.collapsed {
  padding: 3.774em 0 0!important;
}

.js fieldset.collapsed .fieldset-legend a:before {
  transform:
    translate(0, 0.125em)
    rotate(-90deg);
}
.js[dir="rtl"] fieldset.collapsed .fieldset-legend a:before {
  transform:
    translate(0, 0.125em)
    rotate(90deg);
}