Twenty Twelve uses invalid linear-gradient() syntax

[vrubleg]

Twenty Twelve theme uses such construction for gradients:

	background-image: -moz-linear-gradient(top, #f4f4f4, #e6e6e6);
	background-image: -ms-linear-gradient(top, #f4f4f4, #e6e6e6);
	background-image: -webkit-linear-gradient(top, #f4f4f4, #e6e6e6);
	background-image: -o-linear-gradient(top, #f4f4f4, #e6e6e6);
	background-image: linear-gradient(top, #f4f4f4, #e6e6e6);

Notice this obsolete syntax:

linear-gradient(top, #f4f4f4, #e6e6e6)

And compare it to the specification:
​https://developer.mozilla.org/en-US/docs/Web/CSS/linear-gradient

It has to use "to" before the direction. So, it has to be:

linear-gradient(to bottom, #f4f4f4, #e6e6e6)

or like this:

linear-gradient(180deg, #f4f4f4, #e6e6e6)

It works now just because browsers parse this property from prefixed linear gradient, and prefixed linear gradient allows to omit "to", and it is actually parsed as "from". It will stop work when support of obsolete prefixed properties will be removed. And we have to replace all "top" to "to bottom" or to "180deg".

[vrubleg]

MDN even provides ​a similar example:

.grad { 
  background-color: #F07575; /* fallback color if gradients are not supported */
  background-image: -webkit-linear-gradient(top, hsl(0, 80%, 70%), #bada55); /* For Chrome 25 and Safari 6, iOS 6.1, Android 4.3 */
  background-image:    -moz-linear-gradient(top, hsl(0, 80%, 70%), #bada55); /* For Firefox (3.6 to 15) */
  background-image:      -o-linear-gradient(top, hsl(0, 80%, 70%), #bada55); /* For old Opera (11.1 to 12.0) */ 
  background-image:         linear-gradient(to bottom, hsl(0, 80%, 70%), #bada55); /* Standard syntax; must be last */
}

Yeah, and maybe it will be better to remove all prefixed versions. All browsers support it for years, even IE10 and IE11 support it. No any reason to keep this obsolete stuff.

[vrubleg]

Also I figured out that this huge style isn't used at all:

.comments-area article header h4 {
	position: absolute;
	top: 0;
	right: 0;
	padding: 6px 12px;
	padding: 0.428571429rem 0.857142857rem;
	font-size: 12px;
	font-size: 0.857142857rem;
	font-weight: normal;
	color: #fff;
	background-color: #0088d0;
	background-repeat: repeat-x;
	background-image: -moz-linear-gradient(top, #009cee, #0088d0);
	background-image: -ms-linear-gradient(top, #009cee, #0088d0);
	background-image: -webkit-linear-gradient(top, #009cee, #0088d0);
	background-image: -o-linear-gradient(top, #009cee, #0088d0);
	background-image: linear-gradient(top, #009cee, #0088d0);
	border-radius: 3px;
	border: 1px solid #007cbd;
}

So, linear gradients are really used only for buttons (in 3 places). That's it.

Also the author forgot to put "button," here:

.menu-toggle,
input[type="submit"],
input[type="button"],
input[type="reset"],
article.post-password-required input[type=submit],
.bypostauthor cite span {

It has to be just after the ".menu-toggle,", like in the next styles:

.menu-toggle:hover,
.menu-toggle:focus,
button:hover,
input[type="submit"]:hover,
input[type="button"]:hover,
input[type="reset"]:hover,
article.post-password-required input[type=submit]:hover {

And:

.menu-toggle:active,
.menu-toggle.toggled-on,
button:active,
input[type="submit"]:active,
input[type="button"]:active,
input[type="reset"]:active {

And the last. In the first style for buttons this part is useless:

.bypostauthor cite span

It is overwritten completely in further CSS rules which reset all values, remove shadow, etc. So, the same thing could be done without this unnecessary redundancy.

[sabernhardt]

adding button element to basic button style rules

[sabernhardt]

Thanks for the report, and sorry for the delayed response.

Since opening this ticket, the gradient syntax was corrected in changeset:45124.

Changeset r22899 applied the gradient to button element in its hover/focus/active states, though not to the button element itself. For consideration, 40940.patch​ adds that. This would alter the styling for at least the search block's submit button and custom buttons (in Custom HTML or Classic blocks, or within a child theme template).

I did not include changes for the following, but of course they still might be worth editing.

1. Removing vendor prefixes: Yes, these are probably all unnecessary at this point. For comparison, Twenty Thirteen only has the -webkit prefix, and Twenty Seventeen includes others again. Twenty Twenty and Twenty Twenty-One do not include prefixes for gradients.
2. Removing the .comments-area article header h4 style: I cannot find an h4 heading in the comments section, even in the ​original functions.php file.
3. Editing .bypostauthor cite span: I think this part would be better to leave as it is. The second set overrides almost all properties from the first, but it maintains the font-size and line-height. (Adding those three lines could negate the benefit of removing the three that reset.)