refactor: splitting out pcss files more to be more atomically organized

.editorconfig

@@ -8,5 +8,5 @@ indent_style = space
 insert_final_newline = true
 trim_trailing_whitespace = true
 
-[*.{css,less,sass,scss,feature}]
+[*.{css,pcss,less,sass,scss,feature}]
 indent_size = 2

css/base/html-body.pcss

@@ -0,0 +1,35 @@
+/* html & body */
+
+html,
+body {
+  margin: 0;
+  padding: 0;
+}
+
+html {
+  box-sizing: border-box;
+  min-width: 300px;
+  -ms-overflow-style: scrollbar;
+  overflow-x: hidden;
+  overflow-y: scroll;
+  background-color: var(--html-background-color, white);
+  -webkit-tap-highlight-color: rgba(0, 0, 0, 0);
+}
+
+*,
+*::before,
+*::after {
+  box-sizing: inherit;
+}
+
+body {
+  background-color: var(--body-background-color, white);
+}
+
+/*
+IE10+ doesn't honor `<meta name="viewport">` in some cases
+source: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_reboot.scss
+*/
+@-ms-viewport {
+  width: device-width;
+}

css/base/interactive-elements.pcss

@@ -0,0 +1,29 @@
+/* interactive elements */
+
+/*
+avoid 300ms click delay on touch devices that support the `touch-action` CSS property
+source: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_reboot.scss
+*/
+@custom-selector :--touch-action-element
+a,
+area,
+button,
+[role="button"],
+input:not([type="range"]),
+label,
+select,
+summary,
+textarea;
+
+:--touch-action-element {
+  touch-action: manipulation;
+}
+
+/*
+suppress the focus outline on elements that cannot be accessed via keyboard
+this prevents an unwanted focus outline from appearing around elements that might still respond to pointer events.
+source: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_reboot.scss
+ */
+[tabindex="-1"]:focus {
+  outline: none !important;
+}

css/base/semantic-elements.pcss

@@ -0,0 +1,19 @@
+/* semantic elements */
+
+/* fix display of HTML5 structural elements in older browsers */
+@custom-selector :--semantic-element
+  article,
+  aside,
+  dialog,
+  figcaption,
+  figure,
+  footer,
+  header,
+  hgroup,
+  main,
+  nav,
+  section;
+
+:--semantic-element {
+  display: block;
+}