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HTML 

              
                <details class="details0">
  <summary>(0) CSS Transition All - Summary</summary>
  Background color animates, but height doesn't. You can't animate to an auto height, but a specific height doesn't (or, at least, it didn't in 2013) help. See #3.
</details>

<details class="details1">
  <summary>(1) CSS Transition (Auto) Height - Summary</summary>
  No wrapper, no heights set. No animation. You can't animate to an auto height, but a specific height doesn't (or, at least, it didn't in 2013) help. See #3.
</details>

<details class="details2">
  <summary>(2) CSS Transition (Auto) Height of Wrapper - Summary</summary>
  <div class="contents">Added wrapper, height: auto. Still no animation. You can't animate to an auto height, but a specific height doesn't (or, at least, it didn't in 2013) help. See #3.</div>
</details>

<details class="details3">
  <summary>(3) CSS Transition (Specific) Height - Summary</summary>
  No wrapper, specific heights set. Still no animation. Or, at least, not when this was originally created in 2013. As of 2016 (maybe earlier) it animates, albeit not ideally, in Chrome, Firefox, Opera, and Safari! <a href="https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/6261266-details-summary-elements">IE and Edge don't support summary and details at all, remember</a>.
</details>

<details class="details4">
  <summary>(4) CSS Transition (Specific) Height of Wrapper - Summary</summary>
  <div class="contents">
    Wrapper with specific heights set. No animation. Still.
  </div>
</details>

<details class="details5">
  <summary>(5) CSS fadeInDown Animation - Summary</summary>
  Try using <a href="https://daneden.me/">Dan Eden's</a> fadeInDown to maybe fake it a little.  Animation.
</details>

<details class="details6">
  <summary>(6) CSS font-size Animation - Summary</summary>
  Try using <a href="http://stackoverflow.com/users/923745/steven-vachon">Seven Vachon's</a> comment regarding font-size technique to maybe fake it a little differently. Animation.
</details>

<details class="details7">
  <summary>(7) CSS font-size Animation - Summary</summary>
  <div class="content">
    Try using <a href="http://stackoverflow.com/users/923745/steven-vachon">Seven Vachon's</a> comment regarding font-size technique to maybe fake it a little differently. Animation.</div>
</details>

<div class="below">
  Example of contents below that would get pushed down or pulled up.
</div>
!

CSS 

              
                /* everything */
.details0 { 
  transition: all 0.5s linear;
}

/* height only, no heights */
.details1 {
  transition: height 1s ease;
}

/* height only, auto heights */
.details2, .details2 .contents {
  transition: height 1s ease;
}
.details2:not([open]) { height: auto; }
.details2[open] { height: auto; }

/* specific heights */
.details3 {
  transition: height 1s ease;
}
.details3:not([open]) { height: 1.25em; }
.details3[open] { height: 3.75em; }

/* specific heights with wrapper */
.details4 .contents {
  transition: height 1s ease;
}
.details4:not([open]) .contents { height: 0; }
.details4[open] .contents { height: 1.25em; }

/* specific animation */
/* https://daneden.me/animate/ */
@keyframes fadeInDown {
	0% {
		opacity: 0;
		transform: translateY(-1.25em);
	}
	100% {
		opacity: 1;
		transform: translateY(0);
	}
}
.details5[open] {
	animation-name: fadeInDown;
  animation-duration: 0.5s;
}

/* animate font-size (for height: auto) */
@keyframes growAndFadeInFontSize {
	0% {
    font-size: 0;
    opacity: 0;
	}
	100% {
    font-size: 1em;
    opacity: 1;
	}
}
.details6[open] {
	animation-name: growAndFadeInFontSize;
  animation-duration: 500ms;
}

/* add wrapper, continue with font-size */
@keyframes invisiblyGrowFontSize {
	0% {
    font-size: 0;
    opacity: 0;
	}
	100% {
    font-size: 1em;
    opacity: 0;
	}
}
@keyframes fadeIn {
  0% {
    opacity: 0;
  }
	100% {
		opacity: 1;
	}
}
.details7[open] .content {
	animation-name: invisiblyGrowFontSize, fadeIn;
  animation-duration: 500ms, 200ms;
  animation-delay: 0ms, 500ms;
}

/* debugging colors */
details:not([open]) { background: rgba(0,0,255,0.15); }
details[open] { background: rgba(255,0,0,0.15); }
details:hover { background: rgba(0,255,0,0.15); }
.below { background: rgba(255,255,0,0.15); }

/* debugging styles */
html, details, summary { font-family: sans-serif; line-height: 1.25; }
details { margin-bottom: 0.25em; line-height: 1.25; }
!

JS
!
999px

Console