app/services/message_builder.py:88:def validate_html(html: str) -> List[str]:
18:from app.services.message_builder import html_to_pdf, sanitize_preview, validate_html
131:        problems = validate_html(html_body)
136:        problems = [p for p in problems if not p.startswith("Unsubstituted variables")]
137:        if problems:  # validate_html returns a list of problem strings
139:                                detail=f"HTML rejected: {'; '.join(problems)}")
200:    problems = validate_html(res.text)
201:    if problems:  # list of problem strings
203:                            detail=f"Template has errors: {'; '.join(problems)}")