Optional too optional, apparently

Local testing did not reveal that "optional" is incredibly eagerly skipped
if it involves network transit, so successive page loads didn't actually
load the fonts. "fallback" behaved as-predicted (success on next page
load). Some styles set to "swap" to guarantee better readability. "Oops."

Author: workingjubilee

src/librustdoc/html/static/rustdoc.css

@@ -3,12 +3,14 @@
 	font-family: 'Fira Sans';
 	font-style: normal;
 	font-weight: 400;
+	font-display: swap;
 	src: local('Fira Sans'), url("FiraSans-Regular.woff") format('woff');
 }
 @font-face {
 	font-family: 'Fira Sans';
 	font-style: normal;
 	font-weight: 500;
+	font-display: swap;
 	src: local('Fira Sans Medium'), url("FiraSans-Medium.woff") format('woff');
 }
 
@@ -17,18 +19,21 @@
 	font-family: 'Source Serif Pro';
 	font-style: normal;
 	font-weight: 400;
+	font-display: fallback;
 	src: local('Source Serif Pro'), url("SourceSerifPro-Regular.ttf.woff") format('woff');
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: italic;
 	font-weight: 400;
+	font-display: fallback;
 	src: local('Source Serif Pro Italic'), url("SourceSerifPro-It.ttf.woff") format('woff');
 }
 @font-face {
 	font-family: 'Source Serif Pro';
 	font-style: normal;
 	font-weight: 700;
+	font-display: fallback;
 	src: local('Source Serif Pro Bold'), url("SourceSerifPro-Bold.ttf.woff") format('woff');
 }
 
@@ -37,6 +42,7 @@
 	font-family: 'Source Code Pro';
 	font-style: normal;
 	font-weight: 400;
+	font-display: fallback;
 	/* Avoid using locally installed font because bad versions are in circulation:
 	 * see https://github.com/rust-lang/rust/issues/24355 */
 	src: url("SourceCodePro-Regular.woff") format('woff');
@@ -45,6 +51,7 @@
 	font-family: 'Source Code Pro';
 	font-style: normal;
 	font-weight: 600;
+	font-display: swap;
 	src: url("SourceCodePro-Semibold.woff") format('woff');
 }