diff --git a/src/main/python/ttconv/filters/doc/__init__.py b/src/main/python/ttconv/filters/doc/__init__.py
index 4567a21e..d6c21d3c 100644
--- a/src/main/python/ttconv/filters/doc/__init__.py
+++ b/src/main/python/ttconv/filters/doc/__init__.py
@@ -23,13 +23,15 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""Data model filter"""
+"""Collects document instance filters"""
 
 import importlib
 import pkgutil
 import os.path
 import sys
 
+# registers all document instance filters
+
 for importer, package_name, _ in pkgutil.iter_modules([os.path.dirname(__file__)]):
   full_name = f"{__name__}.{package_name}"
   importlib.import_module(full_name)
diff --git a/src/main/python/ttconv/filters/isd/__init__.py b/src/main/python/ttconv/filters/isd/__init__.py
index 88851cf2..22a58522 100644
--- a/src/main/python/ttconv/filters/isd/__init__.py
+++ b/src/main/python/ttconv/filters/isd/__init__.py
@@ -23,7 +23,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""Data model filter"""
+"""Collects ISD filters"""
 
 
 
diff --git a/src/main/python/ttconv/filters/remove_animations.py b/src/main/python/ttconv/filters/remove_animations.py
index 29f1070f..9691472c 100644
--- a/src/main/python/ttconv/filters/remove_animations.py
+++ b/src/main/python/ttconv/filters/remove_animations.py
@@ -23,7 +23,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""Filter removing animations"""
+"""Filter that remove animations"""
 
 import logging
 from typing import Dict, List, Type
@@ -41,6 +41,7 @@ def has_removed_animations(self) -> bool:
     return self._has_removed_animations
 
   def process_element(self, element: ContentElement, recursive = True):
+    """Removes animations from content elements"""
 
     for step in element.iter_animation_steps():
       element.remove_animation_step(step)
diff --git a/src/main/python/ttconv/filters/supported_style_properties.py b/src/main/python/ttconv/filters/supported_style_properties.py
index 6f6c09a4..1ba8e90a 100644
--- a/src/main/python/ttconv/filters/supported_style_properties.py
+++ b/src/main/python/ttconv/filters/supported_style_properties.py
@@ -23,7 +23,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-"""Filter for style properties supported by the output"""
+"""Filters style properties"""
 
 import logging
 from typing import Dict, List, Type
@@ -32,12 +32,13 @@
 from ttconv.style_properties import StyleProperty
 
 class SupportedStylePropertiesFilter:
-  """Filter that remove unsupported style properties"""
+  """Filter that removes unsupported style properties"""
 
   def __init__(self, supported_style_properties: Dict[Type[StyleProperty], List]):
     self.supported_style_properties = supported_style_properties
 
   def process_initial_values(self, doc: ContentDocument):
+    """Removes initial values that target unsupported style properties"""
     for style_prop, value in list(doc.iter_initial_values()):
 
       if style_prop in self.supported_style_properties:
@@ -49,7 +50,7 @@ def process_initial_values(self, doc: ContentDocument):
       doc.put_initial_value(style_prop, None)
 
   def process_element(self, element: ContentElement, recursive = True):
-    """Filter element style properties"""
+    """Removes unsupported style properties from content elements"""
 
     for style_prop in list(element.iter_styles()):