Fix skip links, focus rings, and button semantics for keyboard a11y (#2779)

Co-authored-by: Bartek <xbartoszdobija@gmail.com>

core/scss/_variables.scss

@@ -1006,7 +1006,7 @@ $box-shadow-inset: inset 0 1px 2px rgba($black, 0.075) !default;
 $component-active-color: $white !default;
 $component-active-bg: $primary !default;
 
-// Focus
+// Focus (WCAG 2.2 SC 1.4.11)
 $focus-ring-width: 0.25rem !default;
 $focus-ring-opacity: 0.25 !default;
 $focus-ring-color: color-mix(in srgb, var(--primary) #{math.percentage($focus-ring-opacity)}, transparent) !default;
@@ -1355,7 +1355,7 @@ $carousel-indicators-vertical-margin-inline-end: 1rem !default;
 
 // Close
 $btn-close-width: 1em !default;
-$btn-close-padding-x: 0.25em !default;
+$btn-close-padding-x: 0.875em !default;
 $btn-close-padding-y: $btn-close-padding-x !default;
 $btn-close-color: $body-color !default;
 $btn-close-bg: url("data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 16 16' fill='#{$btn-close-color}'><path d='M.293.293a1 1 0 0 1 1.414 0L8 6.586 14.293.293a1 1 0 1 1 1.414 1.414L9.414 8l6.293 6.293a1 1 0 0 1-1.414 1.414L8 9.414l-6.293 6.293a1 1 0 0 1-1.414-1.414L6.586 8 .293 1.707a1 1 0 0 1 0-1.414'/></svg>") !default;

core/scss/bootstrap/_placeholders.scss

@@ -31,6 +31,10 @@
 .placeholder-glow {
   .placeholder {
     animation: placeholder-glow 2s ease-in-out infinite;
+
+    @media (prefers-reduced-motion: reduce) {
+      animation: none;
+    }
   }
 }
 
@@ -44,6 +48,10 @@
   mask-image: linear-gradient(130deg, $black 55%, rgba(0, 0, 0, (1 - $placeholder-opacity-min)) 75%, $black 95%);
   mask-size: 200% 100%;
   animation: placeholder-wave 2s linear infinite;
+
+  @media (prefers-reduced-motion: reduce) {
+    animation: none;
+  }
 }
 
 @keyframes placeholder-wave {