use dummy emoji font name

Author: wxiaoguang

web_src/css/base.css

@@ -2,8 +2,10 @@
   /* fonts */
   --fonts-proportional: -apple-system, "Segoe UI", system-ui, Roboto, "Helvetica Neue", Arial;
   --fonts-monospace: ui-monospace, SFMono-Regular, "SF Mono", Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace, var(--fonts-emoji);
-  /* "Twemoji Mozilla" emoji font is widely used by browsers like Firefox, Pale Moon, and it is more likely up-to-dated than the system emoji font */
-  --fonts-emoji: "Twemoji Mozilla", "Apple Color Emoji", "Segoe UI Emoji", "Noto Color Emoji";
+  /* GitHub explicitly sets font names like: "Apple Color Emoji", "Segoe UI Emoji", "Noto Color Emoji", "Twemoji Mozilla";
+  Actually "Twemoji Mozilla" emoji font is widely used by browsers like Firefox, Pale Moon, and it is more likely up-to-dated than the system emoji font.
+  So not setting emoji font seems to be the best choice, here we just use a non-existing dummy font name and let browsers choose. */
+  --fonts-emoji: "-emoji-fallback";
   /* font weights - use between 400 and 600 for general purposes. Avoid 700 as it is perceived too bold */
   --font-weight-light: 300;
   --font-weight-normal: 400;