# HTML Email Rendering Order
## CRITICAL: Regex substitution order
When converting Markdown email body to HTML, the order of operations matters:
**WRONG (produces literal `## Title` in HTML):**
1. Convert `\n` to `
` FIRST
2. Then try to match `^## (.+)$` — FAILS because lines are now `
`-delimited
**RIGHT (renders styled `` in HTML):**
1. Regex-substitute headers FIRST (`^## (.+)$` → `\1
` with `re.MULTILINE`)
2. Regex-substitute horizontal rules (`^---$` → `
`)
3. Convert bold (`**text**` → ``)
4. Convert newlines LAST (`\n` → `
`)
```python
import re
html = body
html = re.sub(r'^## (.+)$', r'\1
', html, flags=re.MULTILINE)
html = re.sub(r'^### (.+)$', r'\1
', html, flags=re.MULTILINE)
html = re.sub(r'^---$', r'
', html, flags=re.MULTILINE)
html = re.sub(r'\*\*(.+?)\*\*', r'\1', html)
html = html.replace('\n', '
')
```
## Always use multipart/alternative
- Plain text part first (keeps raw markdown — degrades gracefully)
- HTML part second (email clients render the last part they understand)
- Wrapped in full `` with inline `