Follow HTML5 spec update

See also: https://html.spec.whatwg.org/multipage/scripting.html#the-slot-element

after/syntax/css/html5.vim

@@ -1 +1 @@
-syn keyword cssTagName picture rb rtc template
+syn keyword cssTagName picture rb rtc slot template

test/test.css

@@ -818,6 +818,7 @@
 picture,
 rb,
 rtc,
+slot,
 template {
   display: auto;
 }

test/test.html

@@ -901,6 +901,7 @@
 picture,
 rb,
 rtc,
+slot,
 template {
   display: auto;
 }