How can I render HTML-file content to specific tag/word into a template.docx file (word replacement)?

Question

How can I copy the content of an HTML file (data_1.html") to a specific location in a pre-made "template.docx" and then save it as "output.docx" with python?

I know, that it is possible with this simple Code to convert html to docx perfectly included formats like (bold,underline,italic):

from htmldocx import HtmlToDocx

new_parser = HtmlToDocx()
new_parser.parse_html_file(input_html_file_path, output_docx_file_path)

But somehow I'm not able to replace a word/tag with the content, like it is kind of possible with "python-docx-template". There it is easy to define a word, that replaces specific text within a template like here:

from docxtpl import DocxTemplate

doc = DocxTemplate("my_word_template.docx")
context = { 'company_name' : "World company" }
doc.render(context)
doc.save("generated_doc.docx")

But how is it possible to define the content of a HTML-file that replaces the word in a "template.docx" file?

When I try it like this...:

from docxtpl import DocxTemplate

doc = DocxTemplate("template.docx")
data1 = open("data_1.html", "r")
context = { 'MONTAG' : data1 }
doc.render(context)
doc.save("output.docx")

...and try to open the "output.docx", I get an error like "Error opening file in Word"

Here is an example of how I would like it to work:

html_to_docx_example