Try to add Link header to preload resources

src/application-globals.rkt

@@ -30,7 +30,8 @@
            html-writing))
 
 (define always-headers
-  (list (header #"Referrer-Policy" #"same-origin"))) ; header to not send referers to fandom
+  (list (header #"Referrer-Policy" #"same-origin") ; header to not send referers to fandom
+        (header #"Link" (string->bytes/latin-1 link-header))))
 (define timeouts (easy:make-timeout-config #:lease 5 #:connect 5))
 
 (define (application-footer source-url #:license [license-in #f])

src/static-data.rkt

@@ -1,14 +1,16 @@
 #lang typed/racket/base
 (require racket/path
-         racket/runtime-path)
+         racket/runtime-path
+         racket/string)
 
 (provide
- get-static-url)
+ get-static-url
+ link-header)
 
 (define-runtime-path path-static "../static")
 
 (define static-data
-  (for/hash ([f (directory-list path-static)]) : (Immutable-HashTable Path Nonnegative-Integer)
+  (for/hash : (Immutable-HashTable Path Nonnegative-Integer)([f (directory-list path-static)])
     (define built (simple-form-path (build-path path-static f)))
     (values built (file-or-directory-modify-seconds built))))
 
@@ -18,3 +20,14 @@
                                          path-or-filename
                                          (build-path path-static path-or-filename))))
   (format "/static/~a?t=~a" (file-name-from-path the-path) (hash-ref static-data the-path)))
+
+(: link-header String)
+(define link-header
+  (let* ([with-t '("main.css")]
+         [with-t-full (map get-static-url with-t)]
+         [without-t '("preact.js" "source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-regular.woff2")]
+         [without-t-full (map (λ (path) (format "/static/~a" path)) without-t)])
+    (string-join
+     (for/list : (Listof String) ([full-path : String (append with-t-full without-t-full)])
+       (format "<~a>; rel=\"preload\"" full-path))
+     ", ")))

static/main.css

@@ -386,7 +386,7 @@ figcaption, .lightbox-caption, .thumbcaption {
   font-style: normal;
   font-weight: 400;
   font-display: swap;
-  src: local(""),
+  src: local("Source Sans Pro"),
        url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-regular.woff2") format("woff2");
 }
 @font-face {
@@ -394,22 +394,19 @@ figcaption, .lightbox-caption, .thumbcaption {
   font-style: italic;
   font-weight: 400;
   font-display: swap;
-  src: local(""),
-       url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-italic.woff2") format("woff2");
+  src: url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-italic.woff2") format("woff2");
 }
 @font-face {
   font-family: "Source Sans Pro";
   font-style: normal;
   font-weight: 700;
   font-display: swap;
-  src: local(""),
-       url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-700.woff2") format("woff2");
+  src: url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-700.woff2") format("woff2");
 }
 @font-face {
   font-family: "Source Sans Pro";
   font-style: italic;
   font-weight: 700;
   font-display: swap;
-  src: local(""),
-       url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-700italic.woff2") format("woff2");
+  src: url("/static/source-sans-pro-v21-vietnamese_latin-ext_latin_greek-ext_greek_cyrillic-ext_cyrillic-700italic.woff2") format("woff2");
 }