Implemented the scripts' deferral on the Page protocol in the Infrastructure package.

Packages/Infrastructure/Tests/Cases/Public/Protocols/PageTests.swift

@@ -21,7 +21,7 @@ struct PageTests {
         #expect(html.contains(#"name="viewport""#))
         #expect(html.contains(#"<meta name="stub" content="marker">"#))
         #expect(html.contains(#"<link rel="stylesheet" href="/css/stub.css">"#))
-        #expect(html.contains(#"<script src="/js/stub.js"></script>"#))
+        #expect(html.contains(#"<script defer src="/js/stub.js"></script>"#))
         #expect(html.contains("Stub content"))
     }
 
@@ -38,13 +38,17 @@ struct PageTests {
     }
 
     @Test
-    func `renders the scripts after the content`() throws {
+    func `renders the scripts deferred in the head, after the stylesheets`() throws {
         let html = StubPage().render()
 
-        let content = try #require(html.range(of: "Stub content"))
+        let stylesheet = try #require(html.range(of: "/css/stub.css"))
         let script = try #require(html.range(of: "/js/stub.js"))
+        let content = try #require(html.range(of: "Stub content"))
 
-        #expect(content.lowerBound < script.lowerBound)
+        // Deferred head scripts start downloading during head parsing but still execute, in order, only after the
+        // document is parsed — the semantics end-of-body tags gave, minus the late download start.
+        #expect(stylesheet.lowerBound < script.lowerBound)
+        #expect(script.lowerBound < content.lowerBound)
     }
 
     @Test