class_docstr: Add Markdown parsing in HTML to Markdown class.
docstr: Register extension instances. 
code: def extendMarkdown(self, md):
      """Register extension instances."""
  
      # Replace raw HTML preprocessor
      md.preprocessors.register(
          HtmlBlockPreprocessor(md), "html_block", 20
      )
      # Add `blockprocessor` which handles the placeholders for `etree` elements
      md.parser.blockprocessors.register(
          MarkdownInHtmlProcessor(md.parser), "markdown_block", 105
      )
      # Replace raw HTML postprocessor
      md.postprocessors.register(
          MarkdownInHTMLPostprocessor(md), "raw_html", 30
      )
  
imports: from __future__ import annotations
  from .None import Extension
  from ..blockprocessors import BlockProcessor
  from ..preprocessors import Preprocessor
  from ..postprocessors import RawHtmlPostprocessor
  from ..None import util
  from ..htmlparser import HTMLExtractor, blank_line_re
  import xml.etree.ElementTree
  from typing import TYPE_CHECKING, Literal, Mapping
label_desc: Use image node_id 1 for calling the MarkdownInHtmlExtension obj's underlying member method code with example usage: obj.extendMarkdown(md) without return types