feat(fs): Implement loading HTML from http.FileSystem (#4053)

* Implement loading HTML from http.FileSystem

* Add OnlyHTMLFS test

* Move OnlyHTMLFS to internal and add test

internal/fs/fs.go

@@ -0,0 +1,22 @@
+package fs
+
+import (
+	"io/fs"
+	"net/http"
+)
+
+// FileSystem implements an [fs.FS].
+type FileSystem struct {
+	http.FileSystem
+}
+
+// Open passes `Open` to the upstream implementation and return an [fs.File].
+func (o FileSystem) Open(name string) (fs.File, error) {
+	f, err := o.FileSystem.Open(name)
+
+	if err != nil {
+		return nil, err
+	}
+
+	return fs.File(f), nil
+}

internal/fs/fs_test.go

@@ -0,0 +1,49 @@
+package fs
+
+import (
+	"errors"
+	"net/http"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+type mockFileSystem struct {
+	open func(name string) (http.File, error)
+}
+
+func (m *mockFileSystem) Open(name string) (http.File, error) {
+	return m.open(name)
+}
+
+func TesFileSystem_Open(t *testing.T) {
+	var testFile *os.File
+	mockFS := &mockFileSystem{
+		open: func(name string) (http.File, error) {
+			return testFile, nil
+		},
+	}
+	fs := &FileSystem{mockFS}
+
+	file, err := fs.Open("foo")
+
+	require.NoError(t, err)
+	assert.Equal(t, testFile, file)
+}
+
+func TestFileSystem_Open_err(t *testing.T) {
+	testError := errors.New("mock")
+	mockFS := &mockFileSystem{
+		open: func(_ string) (http.File, error) {
+			return nil, testError
+		},
+	}
+	fs := &FileSystem{mockFS}
+
+	file, err := fs.Open("foo")
+
+	require.ErrorIs(t, err, testError)
+	assert.Nil(t, file)
+}