diff --git a/check50/renderer/_renderers.py b/check50/renderer/_renderers.py
index 0ddc252a..0d42244b 100644
--- a/check50/renderer/_renderers.py
+++ b/check50/renderer/_renderers.py
@@ -14,11 +14,56 @@ def to_html(slug, results, version):
         content = f.read()
 
     template = jinja2.Template(
-        content, autoescape=jinja2.select_autoescape(enabled_extensions=("html",)))
-    html = template.render(slug=slug, results=results, version=version)
+        content, autoescape=jinja2.select_autoescape(enabled_extensions=("html",))
+    )
+
+    html = template.render(
+        slug=slug,
+        results=results,
+        version=version,
+        fmt_special_chars=_fmt_special_chars,
+        color="808080" # RGB (128, 128, 128)
+    )
 
     return html
 
+def _fmt_special_chars(txt, color):
+    """Converts a plaintext string into a string of HTML elements that highlights special chars."""
+    def highlight_char(char, color):
+        """Highlights and escapes a char."""
+        return f"<span style='background-color:{color}; cursor: help;' title='This is an invisible or trailing special character.'>{repr(char)[1:-1]}</span>"
+
+    # We'd like to interpret whitespace (ws) as HTML in only these specific cases:
+    ws_to_html = {
+        "\n": "<br/>",
+        " ": "&nbsp;",
+    }
+    fmtted_txt = []
+
+    for i, char in enumerate(txt):
+        is_last = i == len(txt) - 1
+
+        if not char.isprintable() and char not in ws_to_html:
+            # Most special invisible characters, excluding those in ws_to_html, are highlighted
+            fmtted_txt.append(highlight_char(char, color))
+        elif char in ws_to_html:
+            # If there's a trailing whitespace character, we highlight it
+            if is_last:
+                # Spaces aren't normally highlightable, so we convert to nbsp.
+                if char == ' ':
+                    char = ws_to_html[char]
+
+                fmtted_txt.append(highlight_char(char, color))
+            else:
+                # Certain special chars are interpreted in HTML, without escaping or highlighting
+                fmtted_txt.append(ws_to_html[char])
+        else:
+            # Non-special characters are unchanged
+            fmtted_txt.append(char)
+
+    # Return the text as a string of plaintext + html elements
+    return ''.join(fmtted_txt)
+
 
 def to_json(slug, results, version):
     return json.dumps({"slug": slug, "results": results, "version": version}, indent=4)
diff --git a/check50/renderer/templates/results.html b/check50/renderer/templates/results.html
index 6a51efa8..40b58d7f 100644
--- a/check50/renderer/templates/results.html
+++ b/check50/renderer/templates/results.html
@@ -86,7 +86,7 @@ <h3 style="color:orange">:| {{ check.description }}</h3>
                                               {% else %}
                                                   {% set expected = check.cause.expected | e %}
                                               {% endif %}
-                                              {{ expected | replace(" ", "&nbsp;") | replace("\n", "<br/>") }}
+                                              {{ fmt_special_chars(expected, color) }}
                                           {% endautoescape %}
                                       </samp>
                                       </div>
@@ -102,12 +102,13 @@ <h3 style="color:orange">:| {{ check.description }}</h3>
                                               {% else %}
                                                   {% set actual = check.cause.actual | e %}
                                               {% endif %}
-                                              {{ actual | replace(" ", "&nbsp;") | replace("\n", "<br/>") }}
+                                              {{ fmt_special_chars(actual, color) }}
                                           {% endautoescape %}
                                       </samp>
                                       </div>
                                   </div>
                               </div>
+                              <br/>
                           {% endif %}
 
                           {# Missing if there was one #}