diff --git a/sanity_html/dataclasses.py b/sanity_html/dataclasses.py
index 0ebe8f5..24cf457 100644
--- a/sanity_html/dataclasses.py
+++ b/sanity_html/dataclasses.py
@@ -3,7 +3,7 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, cast
 
-from sanity_html.utils import get_marker_definitions
+from sanity_html.utils import get_default_marker_definitions
 
 if TYPE_CHECKING:
     from typing import Literal, Optional, Tuple, Type, Union
@@ -44,7 +44,7 @@ class Block:
     listItem: Optional[Literal['bullet', 'number', 'square']] = None
     children: list[dict] = field(default_factory=list)
     markDefs: list[dict] = field(default_factory=list)
-    marker_definitions: dict[str, Type[MarkerDefinition]] = field(init=False)
+    marker_definitions: dict[str, Type[MarkerDefinition]] = field(default_factory=dict)
     marker_frequencies: dict[str, int] = field(init=False)
 
     def __post_init__(self) -> None:
@@ -54,7 +54,9 @@ def __post_init__(self) -> None:
         To make handling of span `marks` simpler, we define marker_definitions as a dict, from which
         we can directly look up both annotation marks or decorator marks.
         """
-        self.marker_definitions = get_marker_definitions(self.markDefs)
+        marker_definitions = get_default_marker_definitions(self.markDefs)
+        marker_definitions.update(self.marker_definitions)
+        self.marker_definitions = marker_definitions
         self.marker_frequencies = self._compute_marker_frequencies()
 
     def _compute_marker_frequencies(self) -> dict[str, int]:
diff --git a/sanity_html/renderer.py b/sanity_html/renderer.py
index 7c4c498..bfdfd8e 100644
--- a/sanity_html/renderer.py
+++ b/sanity_html/renderer.py
@@ -9,7 +9,9 @@
 from sanity_html.utils import get_list_tags, is_block, is_list, is_span
 
 if TYPE_CHECKING:
-    from typing import Dict, List, Optional, Union
+    from typing import Callable, Dict, List, Optional, Type, Union
+
+    from sanity_html.marker_definitions import MarkerDefinition
 
 
 # TODO: Let user pass custom code block definitions/plugins
@@ -19,8 +21,15 @@
 class SanityBlockRenderer:
     """HTML renderer for Sanity block content."""
 
-    def __init__(self, blocks: Union[list[dict], dict]) -> None:
+    def __init__(
+        self,
+        blocks: Union[list[dict], dict],
+        custom_marker_definitions: dict[str, Type[MarkerDefinition]] = None,
+        custom_serializers: dict[str, Callable[[dict, Optional[Block], bool], str]] = None,
+    ) -> None:
         self._wrapper_element: Optional[str] = None
+        self._custom_marker_definitions = custom_marker_definitions or {}
+        self._custom_serializers = custom_serializers or {}
 
         if isinstance(blocks, dict):
             self._blocks = [blocks]
@@ -38,7 +47,8 @@ def render(self) -> str:
         for node in self._blocks:
 
             if list_nodes and not is_list(node):
-                result += self._render_list(list_nodes)
+                tree = self._normalize_list_tree(list_nodes, Block(**node))
+                result += ''.join([self._render_node(n, Block(**node), list_item=True) for n in tree])
                 list_nodes = []  # reset list_nodes
 
             if is_list(node):
@@ -48,7 +58,11 @@ def render(self) -> str:
             result += self._render_node(node)  # render non-list nodes immediately
 
         if list_nodes:
-            result += self._render_list(list_nodes)
+            tree = self._normalize_list_tree(list_nodes, Block(**node))
+            result += ''.join(
+                self._render_node(n, Block(**node), list_item=True) for n in tree
+            )
+
 
         result = result.strip()
 
@@ -64,8 +78,11 @@ def _render_node(self, node: dict, context: Optional[Block] = None, list_item: b
         :param context: Optional context. Spans are passed with a Block instance as context for mark lookups.
         :param list_item: Whether we are handling a list upstream (impacts block handling).
         """
-        if is_block(node):
-            block = Block(**node)
+        if is_list(node):
+            block = Block(**node, marker_definitions=self._custom_marker_definitions)
+            return self._render_list(block, context)
+        elif is_block(node):
+            block = Block(**node, marker_definitions=self._custom_marker_definitions)
             return self._render_block(block, list_item=list_item)
 
         elif is_span(node):
@@ -78,7 +95,8 @@ def _render_node(self, node: dict, context: Optional[Block] = None, list_item: b
 
             assert context  # this should be a cast
             return self._render_span(span, block=context)  # context is span's outer block
-
+        elif custom_serializer := self._custom_serializers.get(node.get('_type', '')):
+            return custom_serializer(node, context, list_item)
         else:
             print('Unexpected code path 👺')  # noqa: T001 # TODO: Remove after thorough testing
             return ''
@@ -120,40 +138,87 @@ def _render_span(self, span: Span, block: Block) -> str:
 
         return result
 
-    def _render_list(self, nodes: list) -> str:
-        result, tag_dict = '', {}
-        for index, node in enumerate(nodes):
-
-            current_level = node['level']  # 1
-            prev_level = nodes[index - 1]['level'] if index > 0 else 0  # default triggers first condition below
+    def _render_list(self, node: Block, context: Optional[Block]) -> str:
+        assert node.listItem
+        head, tail = get_list_tags(node.listItem)
+        result = head
+        for child in node.children:
+            result += f'<li>{self._render_block(Block(**child), True)}</li>'
+        result += tail
+        return result
 
-            list_item = node.pop('listItem')  # popping this attribute lets us call render_node for non-list handling
-            node_inner_html = '<li>' + ''.join(list(self._render_node(node, list_item=True))) + '</li>'
+    def _normalize_list_tree(self, nodes: list, block: Block) -> list[dict]:
+        tree = []
 
-            if current_level > prev_level:
-                list_tags = get_list_tags(list_item)
-                result += list_tags[0]
-                result += node_inner_html
-                tag_dict[current_level] = list_tags[1]
+        current_list = None
+        for node in nodes:
+            if not is_block(node):
+                tree.append(node)
+                current_list = None
                 continue
 
-            elif current_level == prev_level:
-                result += node_inner_html
+            if current_list is None:
+                current_list = self._list_from_block(node)
+                tree.append(current_list)
                 continue
 
-            elif current_level < prev_level:
-                result += node_inner_html
-                result += tag_dict.pop(prev_level)
+            if node.get('level') == current_list['level'] and node.get('listItem') == current_list['listItem']:
+                current_list['children'].append(node)
                 continue
 
-            else:
-                print('Unexpected code path 🕵🏻‍')  # noqa: T001 # TODO: Remove or alter when done testing
+            if node.get('level') > current_list['level']:
+                new_list = self._list_from_block(node)
+                current_list['children'][-1]['children'].append(new_list)
+                current_list = new_list
+                continue
 
-        # there should be one or more tag in the dict for us to close off
-        for value in tag_dict.values():
-            result += value
+            if node.get('level') < current_list['level']:
+                parent = self._find_list(tree[-1], level=node.get('level'), list_item=node.get('listItem'))
+                if parent:
+                    current_list = parent
+                    current_list['children'].append(node)
+                    continue
+                current_list = self._list_from_block(node)
+                tree.append(current_list)
+                continue
 
-        return result
+            if node.get('listItem') != current_list['listItem']:
+                match = self._find_list(tree[-1], level=node.get('level'))
+                if match and match['listItem'] == node.get('listItem'):
+                    current_list = match
+                    current_list.children.append(node)
+                    continue
+                current_list = self._list_from_block(node)
+                tree.append(current_list)
+                continue
+            # TODO: Warn
+            tree.append(node)
+
+        return tree
+
+    def _find_list(self, root_node: dict, level: int, list_item: Optional[str] = None) -> Optional[dict]:
+        filter_on_type = isinstance(list_item, str)
+        if (
+            root_node.get('_type') == 'list'
+            and root_node.get('level') == level
+            and (filter_on_type and root_node.get('listItem') == list_item)
+        ):
+            return root_node
+
+        children = root_node.get('children')
+        if children:
+            return self._find_list(children[-1], level, list_item)
+
+        return None
+
+    def _list_from_block(self, block: dict) -> dict:
+        return {
+            '_type': 'list',
+            '_key': f'${block["_key"]}-parent',
+            'level': block.get('level'),
+            'listItem': block['listItem'],
+            'children': [block],
+        }
 
 
 def render(blocks: List[Dict]) -> str:
diff --git a/sanity_html/utils.py b/sanity_html/utils.py
index d3c6d5e..1c75293 100644
--- a/sanity_html/utils.py
+++ b/sanity_html/utils.py
@@ -10,7 +10,7 @@
     from sanity_html.marker_definitions import MarkerDefinition
 
 
-def get_marker_definitions(mark_defs: list[dict]) -> dict[str, Type[MarkerDefinition]]:
+def get_default_marker_definitions(mark_defs: list[dict]) -> dict[str, Type[MarkerDefinition]]:
     """
     Convert JSON definitions to a map of marker definition renderers.
 
@@ -38,7 +38,7 @@ def is_span(node: dict) -> bool:
 
 def is_block(node: dict) -> bool:
     """Check whether a node is a block node."""
-    return node.get('_type') == 'block' and 'listItem' not in node
+    return node.get('_type') == 'block'
 
 
 def get_list_tags(list_item: str) -> tuple[str, str]:
diff --git a/tests/test_upstream_suite.py b/tests/test_upstream_suite.py
index 2a33893..de16467 100644
--- a/tests/test_upstream_suite.py
+++ b/tests/test_upstream_suite.py
@@ -1,9 +1,50 @@
 import json
+import re
 from pathlib import Path
+from typing import Optional
 
 import pytest
 
 from sanity_html import render
+from sanity_html.dataclasses import Block
+from sanity_html.marker_definitions import LinkMarkerDefinition, MarkerDefinition
+from sanity_html.renderer import SanityBlockRenderer
+
+
+def fake_image_serializer(node: dict, context: Optional[Block], list_item: bool):
+    assert node['_type'] == 'image'
+    if 'url' in node['asset']:
+        image_url = node['asset']['url']
+    else:
+        project_id = '3do82whm'
+        dataset = 'production'
+        asset_ref: str = node['asset']['_ref']
+        image_path = asset_ref[6:].replace('-jpg', '.jpg').replace('-png', '.png')
+        image_url = f'https://cdn.sanity.io/images/{project_id}/{dataset}/{image_path}'
+
+    if 'crop' in node and 'hotspot' in node:
+        crop = node['crop']
+        hotspot = node['hotspot']
+        size_match = re.match(r'.*-(\d+)x(\d+)\..*', image_url)
+        if size_match:
+            orig_width, orig_height = [int(x) for x in size_match.groups()]
+            rect_x1 = round((orig_width * hotspot['x']) - ((orig_width * hotspot['width']) / 2))
+            rect_y1 = round((orig_height * hotspot['y']) - ((orig_height * hotspot['height']) / 2))
+            rect_x2 = round(orig_width - (orig_width * crop['left']) - (orig_width * crop['right']))
+            rect_y2 = round(orig_height - (orig_height * crop['top']) - (orig_height * crop['bottom']))
+            # These are passed as "imageOptions" upstream.
+            # It's up the the implementor of the serializer to fix this.
+            # We might provide one for images that does something like this, but for now
+            # let's just make the test suite pass
+            width = 320
+            height = 240
+
+            image_url += f'?rect={rect_x1},{rect_y1},{rect_x2},{rect_y2}&amp;w={width}&amp;h={height}'
+
+    image = f'<img src="{image_url}"/>'
+    if context:
+        return image
+    return f'<figure>{image}</figure>'
 
 
 def get_fixture(rel_path) -> dict:
@@ -102,21 +143,21 @@ def test_011_basic_numbered_list():
     assert output == expected_output
 
 
-@pytest.mark.unsupported
 def test_012_image_support():
     fixture_data = get_fixture('fixtures/upstream/012-image-support.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+    sbr = SanityBlockRenderer(input_blocks, custom_serializers={'image': fake_image_serializer})
+    output = sbr.render()
     assert output == expected_output
 
 
-@pytest.mark.unsupported
 def test_013_materialized_image_support():
     fixture_data = get_fixture('fixtures/upstream/013-materialized-image-support.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+    sbr = SanityBlockRenderer(input_blocks, custom_serializers={'image': fake_image_serializer})
+    output = sbr.render()
     assert output == expected_output
 
 
@@ -189,7 +230,8 @@ def test_022_inline_node():
     fixture_data = get_fixture('fixtures/upstream/022-inline-nodes.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+    sbr = SanityBlockRenderer(input_blocks, custom_serializers={'image': fake_image_serializer})
+    output = sbr.render()
     assert output == expected_output
 
 
@@ -201,21 +243,21 @@ def test_023_hard_break():
     assert output == expected_output
 
 
-@pytest.mark.unsupported
 def test_024_inline_image():
     fixture_data = get_fixture('fixtures/upstream/024-inline-images.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+    sbr = SanityBlockRenderer(input_blocks, custom_serializers={'image': fake_image_serializer})
+    output = sbr.render()
     assert output == expected_output
 
 
-@pytest.mark.unsupported
 def test_025_image_with_hotspot():
     fixture_data = get_fixture('fixtures/upstream/025-image-with-hotspot.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+    sbr = SanityBlockRenderer(input_blocks, custom_serializers={'image': fake_image_serializer})
+    output = sbr.render()
     assert output == expected_output
 
 
@@ -262,12 +304,19 @@ def test_052_custom_mark():
     assert output == expected_output
 
 
-@pytest.mark.unsupported
 def test_053_override_default_mark():
     fixture_data = get_fixture('fixtures/upstream/053-override-default-marks.json')
     input_blocks = fixture_data['input']
     expected_output = fixture_data['output']
-    output = render(input_blocks)
+
+    class CustomLinkMark(LinkMarkerDefinition):
+        @classmethod
+        def render_prefix(cls, span, marker, context) -> str:
+            result = super().render_prefix(span, marker, context)
+            return result.replace('<a href', '<a class=\"mahlink\" href')
+
+    sbr = SanityBlockRenderer(input_blocks, custom_marker_definitions={'mark1': CustomLinkMark})
+    output = sbr.render()
     assert output == expected_output