diff --git a/cairosvg/helpers.py b/cairosvg/helpers.py
index 65a5331a..edfabad0 100644
--- a/cairosvg/helpers.py
+++ b/cairosvg/helpers.py
@@ -388,3 +388,24 @@ def size(surface, string, reference='xy'):
 
     # Unknown size
     return 0
+
+
+def parse_opacity(string, default=1):
+    """Return the opacity of ``string`` as a float clamped to ``[0, 1]``.
+
+    ``string`` may be a number or a percentage (e.g. ``'50%'``). Any value
+    that cannot be parsed falls back to ``default`` instead of raising, so an
+    invalid ``*-opacity`` attribute never aborts the whole rendering.
+
+    """
+    if string is None:
+        return default
+    string = string.strip()
+    try:
+        if string.endswith('%'):
+            value = float(string[:-1]) / 100
+        else:
+            value = float(string)
+    except ValueError:
+        return default
+    return min(1, max(0, value))
diff --git a/cairosvg/surface.py b/cairosvg/surface.py
index ee7920f7..e60ce6aa 100644
--- a/cairosvg/surface.py
+++ b/cairosvg/surface.py
@@ -14,7 +14,7 @@
     filter_, gradient_or_pattern, linear_gradient, marker, mask, paint_mask,
     parse_all_defs, pattern, prepare_filter, radial_gradient, use)
 from .helpers import (
-    UNITS, PointError, clip_rect, node_format, normalize, paint,
+    UNITS, PointError, clip_rect, node_format, normalize, paint, parse_opacity,
     preserve_ratio, size, transform)
 from .image import image, invert_image
 from .parser import Tree
@@ -322,7 +322,7 @@ def draw(self, node):
         # Find and prepare opacity, masks and filters
         mask = parse_url(node.get('mask')).fragment
         filter_ = parse_url(node.get('filter')).fragment
-        opacity = float(node.get('opacity', 1))
+        opacity = parse_opacity(node.get('opacity'))
 
         if filter_:
             prepare_filter(self, node, filter_)
@@ -412,8 +412,8 @@ def draw(self, node):
                 pass
 
         # Get stroke and fill opacity
-        stroke_opacity = float(node.get('stroke-opacity', 1))
-        fill_opacity = float(node.get('fill-opacity', 1))
+        stroke_opacity = parse_opacity(node.get('stroke-opacity'))
+        fill_opacity = parse_opacity(node.get('fill-opacity'))
         if opacity < 1 and not node.children:
             stroke_opacity *= opacity
             fill_opacity *= opacity
diff --git a/cairosvg/test_api.py b/cairosvg/test_api.py
index 9f1da3f4..f63d5486 100644
--- a/cairosvg/test_api.py
+++ b/cairosvg/test_api.py
@@ -37,6 +37,31 @@ def test_formats(format_name):
     assert content.startswith(MAGIC_NUMBERS[format_name])
 
 
+def _rect_svg(attribute, value):
+    return (
+        '<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10">'
+        f'<rect width="10" height="10" fill="red" {attribute}="{value}"/>'
+        '</svg>').encode('ascii')
+
+
+@pytest.mark.parametrize('attribute', (
+    'opacity', 'fill-opacity', 'stroke-opacity'))
+def test_opacity_percentage(attribute):
+    """Percentage opacities render like their equivalent fractions (#422)."""
+    percentage = svg2png(bytestring=_rect_svg(attribute, '50%'))
+    fraction = svg2png(bytestring=_rect_svg(attribute, '0.5'))
+    assert percentage == fraction
+
+
+@pytest.mark.parametrize('attribute', (
+    'opacity', 'fill-opacity', 'stroke-opacity'))
+def test_opacity_invalid(attribute):
+    """Invalid opacities fall back to opaque instead of crashing (#422)."""
+    invalid = svg2png(bytestring=_rect_svg(attribute, 'null'))
+    opaque = svg2png(bytestring=_rect_svg(attribute, '1'))
+    assert invalid == opaque
+
+
 def read_file(filename):
     """Shortcut to return the whole content of a file as a byte string."""
     with open(filename, 'rb') as file_object: