LibWeb/CSS: Implement add for FontFaceSet

There is still some work to do with some of the underying methods
called inside this method (e.g is_css_connected) but this is a start.

Userland/Libraries/LibWeb/CSS/FontFace.h

@@ -70,6 +70,8 @@ public:
     String line_gap_override() const { return m_line_gap_override; }
     WebIDL::ExceptionOr<void> set_line_gap_override(String const&);
 
+    bool is_css_connected() const { return m_is_css_connected; }
+
     Bindings::FontFaceLoadStatus status() const { return m_status; }
 
     JS::NonnullGCPtr<JS::Promise> load();

Userland/Libraries/LibWeb/CSS/FontFaceSet.cpp

@@ -61,14 +61,41 @@ void FontFaceSet::visit_edges(Cell::Visitor& visitor)
     Base::visit_edges(visitor);
     visitor.visit(m_set_entries);
     visitor.visit(m_ready_promise);
+    visitor.visit(m_loading_fonts);
+    visitor.visit(m_loaded_fonts);
+    visitor.visit(m_failed_fonts);
 }
 
 // https://drafts.csswg.org/css-font-loading/#dom-fontfaceset-add
-JS::NonnullGCPtr<FontFaceSet> FontFaceSet::add(JS::Handle<FontFace> face)
+WebIDL::ExceptionOr<JS::NonnullGCPtr<FontFaceSet>>
+FontFaceSet::add(JS::Handle<FontFace> face)
 {
-    // FIXME: Do the actual spec steps
+    // 1. If font is already in the FontFaceSet’s set entries, skip to the last step of this algorithm immediately.
+    if (m_set_entries->set_has(face))
+        return JS::NonnullGCPtr<FontFaceSet>(*this);
+
+    // 2. If font is CSS-connected, throw an InvalidModificationError exception and exit this algorithm immediately.
+    if (face->is_css_connected()) {
+        return WebIDL::InvalidModificationError::create(realm(), "Cannot add a CSS-connected FontFace to a FontFaceSet"_fly_string);
+    }
+
+    // 3. Add the font argument to the FontFaceSet’s set entries.
     m_set_entries->set_add(face);
-    return *this;
+
+    // 4. If font’s status attribute is "loading"
+    if (face->status() == Bindings::FontFaceLoadStatus::Loading) {
+
+        // 1. If the FontFaceSet’s [[LoadingFonts]] list is empty, switch the FontFaceSet to loading.
+        if (m_loading_fonts.is_empty()) {
+            m_status = Bindings::FontFaceSetLoadStatus::Loading;
+        }
+
+        // 2. Append font to the FontFaceSet’s [[LoadingFonts]] list.
+        m_loading_fonts.append(*face);
+    }
+
+    // 5. Return the FontFaceSet.
+    return JS::NonnullGCPtr<FontFaceSet>(*this);
 }
 
 // https://drafts.csswg.org/css-font-loading/#dom-fontfaceset-delete

Userland/Libraries/LibWeb/CSS/FontFaceSet.h

@@ -27,7 +27,7 @@ public:
 
     JS::NonnullGCPtr<JS::Set> set_entries() const { return m_set_entries; }
 
-    JS::NonnullGCPtr<FontFaceSet> add(JS::Handle<FontFace>);
+    WebIDL::ExceptionOr<JS::NonnullGCPtr<FontFaceSet>> add(JS::Handle<FontFace>);
     bool delete_(JS::Handle<FontFace>);
     void clear();
 
@@ -52,6 +52,10 @@ private:
     JS::NonnullGCPtr<JS::Set> m_set_entries;
     JS::GCPtr<WebIDL::Promise> m_ready_promise; // [[ReadyPromise]]
 
+    Vector<JS::NonnullGCPtr<FontFace>> m_loading_fonts {}; // [[LoadingFonts]]
+    Vector<JS::NonnullGCPtr<FontFace>> m_loaded_fonts {};  // [[LoadedFonts]]
+    Vector<JS::NonnullGCPtr<FontFace>> m_failed_fonts {};  // [[FailedFonts]]
+
     Bindings::FontFaceSetLoadStatus m_status { Bindings::FontFaceSetLoadStatus::Loading };
 };