Fix async-loaded gist embed scripts (#630)

Signed-off-by: Thomas Miceli <tho.miceli@gmail.com>

internal/web/handlers/gist/gist.go

@@ -165,10 +165,39 @@ func escapeJavaScriptContent(htmlContent, cssUrl, themeUrl string) (string, erro
 	}
 
 	js := fmt.Sprintf(`
-        document.write('<link rel="stylesheet" href=%s>');
-        document.write('<link rel="stylesheet" href=%s>');
-        document.write(%s);
-    `,
+(function() {
+    if (!customElements.get('opengist-embed')) {
+        customElements.define('opengist-embed', class extends HTMLElement {
+            constructor() {
+                super();
+                this.attachShadow({ mode: 'open' });
+            }
+            
+            init(css1, css2, content) {
+                this.shadowRoot.innerHTML = %s
+                    <style>
+                        @import url(${css1});
+                        @import url(${css2});
+                        :host { display: block; all: initial; font-family: sans-serif; }
+                    </style>
+                    <div class="container">${content}</div>
+                %s;
+            }
+        });
+    }
+
+    var currentScript = document.currentScript || (function() {
+        var scripts = document.getElementsByTagName('script');
+        return scripts[scripts.length - 1];
+    })();
+
+    const instance = document.createElement('opengist-embed');
+    instance.init(%s, %s, %s);
+ 	currentScript.parentNode.insertBefore(instance, currentScript.nextSibling);
+})();
+`,
+		"`",
+		"`",
 		string(jsonCssUrl),
 		string(jsonThemeUrl),
 		string(jsonContent),

public/css/embed.css

@@ -35,6 +35,14 @@
     --border-width-1: 1px;
 }
 
+@layer base {
+    :host {
+        --tw-border-style: solid;
+        --tw-border-width: 0;
+        display: block;
+    }
+}
+
 .opengist-embed {
     @import "tailwindcss";
     @layer base {
@@ -50,4 +58,4 @@
         @import './ipynb.css';
         @import "./style.css";
     }
-}
+}