diff --git a/format.rkt b/format.rkt
new file mode 100644
index 0000000..f3f00d3
--- /dev/null
+++ b/format.rkt
@@ -0,0 +1,66 @@
+#lang racket
+(module+ test (require rackunit "main.rkt"))
+
+(define (file->html coverage paths)
+  (for/list ([path paths])
+    (define file (file->string path))
+    (define cover (hash-ref coverage path))
+    (define data
+      (let loop ([loc 1] [chars (string->list file)] [mode 'none])
+        (match chars
+          [(list) (mode->end mode)]
+          [(cons c r)
+           (define (loop* me) (loop (add1 loc) r m))
+           (define m (get-mode loc cover))
+           (define encoded (encode-char c))
+           (if (eq? m mode)
+               (cons encoded (loop* mode))
+               (append (mode->end mode)
+                       (mode->start m)
+                       (list encoded)
+                       (loop* m)))])))
+    (apply string data)))
+
+(define (get-mode loc c)
+  (define-values (mode _)
+    (for/fold ([mode 'none] [last-start 0])
+              ([pair c])
+      (match pair
+        [(list m (srcloc _ _ _ start range))
+         (if (and (<= start loc (+ start range))
+                  (or (eq? mode 'none)
+                      (> start last-start)))
+             (values m start)
+             (values mode last-start))])))
+  mode)
+
+(define (encode-char c) c)
+
+(define covered-mode-start "<span style=\"color:green\">")
+(define uncovered-mode-start "<span style=\"color:red\">")
+(define (mode->start mode)
+  (string->list
+   (match mode
+     ['none ""]
+     [#t covered-mode-start]
+     [#f uncovered-mode-start])))
+
+(define mode-end "</span>")
+(define (mode->end mode)
+  (string->list
+   (match mode
+     ['none ""]
+     [_ mode-end])))
+
+(module+ test
+  (define (test file out)
+    (test-files! file)
+    (check-equal? (first (file->html (get-test-coverage) (list file)))
+                  out)
+    (clear-coverage!))
+  (test "tests/basic/prog.rkt"
+        (string-append covered-mode-start
+                       (apply string
+                              (map encode-char
+                                   (string->list (file->string "tests/basic/prog.rkt"))))
+                       mode-end)))