diff --git a/loop/webui/memfs.go b/loop/webui/memfs.go
new file mode 100644
index 0000000..5431862
--- /dev/null
+++ b/loop/webui/memfs.go
@@ -0,0 +1,53 @@
+package webui
+
+import (
+	"bytes"
+	"fmt"
+	"io/fs"
+	"time"
+)
+
+// memFS implements fs.FS in-memory.
+type memFS struct {
+	m map[string][]byte
+}
+
+func (m memFS) Open(name string) (fs.File, error) {
+	b, found := m.m[name]
+	if !found {
+		return nil, fmt.Errorf("esbuild.memFS(%q): %w", name, fs.ErrNotExist)
+	}
+	return &memFile{name: name, Reader: *bytes.NewReader(b)}, nil
+}
+
+func (m memFS) ReadFile(name string) ([]byte, error) {
+	b, found := m.m[name]
+	if !found {
+		return nil, fmt.Errorf("esbuild.memFS.ReadFile(%q): %w", name, fs.ErrNotExist)
+	}
+	return append(make([]byte, 0, len(b)), b...), nil
+}
+
+// memFile implements fs.File in-memory.
+type memFile struct {
+	// embedding is very important here because need more than
+	// Read, we need Seek to make http.ServeContent happy.
+	bytes.Reader
+	name string
+}
+
+func (f *memFile) Stat() (fs.FileInfo, error) { return &memFileInfo{f: f}, nil }
+func (f *memFile) Close() error               { return nil }
+
+var start = time.Now()
+
+type memFileInfo struct {
+	f *memFile
+}
+
+func (i memFileInfo) Name() string       { return i.f.name }
+func (i memFileInfo) Size() int64        { return i.f.Reader.Size() }
+func (i memFileInfo) Mode() fs.FileMode  { return 0o444 }
+func (i memFileInfo) ModTime() time.Time { return start }
+func (i memFileInfo) IsDir() bool        { return false }
+func (i memFileInfo) Sys() any           { return nil }