code: def test_repr_html_hlg_layers():
      pytest.importorskip("jinja2")
      hg = HighLevelGraph(
          {"a": {"a": 1, ("a", 0): 2, "b": 3}, "b": {"c": 4}},
          {"a": set(), "b": set()},
      )
      assert (
          xml.etree.ElementTree.fromstring(hg._repr_html_()) is not None
      )
      for layer in hg.layers.values():
          assert (
              xml.etree.ElementTree.fromstring(layer._repr_html_())
              is not None
          )
  
imports: from __future__ import annotations
  import os
  import threading
  import xml.etree.ElementTree
  from collections.abc import Set
  from concurrent.futures import ThreadPoolExecutor
  import pytest
  import dask
  from dask.blockwise import Blockwise, blockwise_token
  from dask.highlevelgraph import HighLevelGraph, Layer, MaterializedLayer, to_graphviz
  from dask.utils_test import inc
label_desc: Use image node_id 8 for calling a global function with example usage: test_repr_html_hlg_layers() without return types