Layout and styling:

Structure with nested — no flexbox, no grid, no floats, no position. One centered wrapper table, max-width 600px, single-column flow (stacked rows beat side-by-side columns).
Inline EVERY style on the element it styles. A block in
No JavaScript anywhere (universally stripped). No external stylesheets. No web fonts — use email-safe stacks (Arial, Helvetica, Georgia, Verdana, Tahoma, 'Courier New') with generic fallbacks.
Build the visual design out of colored table cells, borders, spacer cells, and type — not images. There is nowhere to host project images from here: a referenced project file will not exist for recipients. If imagery is essential, leave a clearly-marked placeholder cell with alt text and tell the user to swap in a hosted https URL before sending.
Buttons are "bulletproof": a padded with bgcolor and inline border-radius, the filling it with display:block and inline color — never an image, never a styled .

Client quirks that matter:

Outlook (Word engine): give every table/cell explicit widths; set line-height with mso-line-height-rule:exactly; wrap Outlook-only fixes in conditionals.
Gmail clips messages beyond ~100KB of HTML — stay well under.
Add and pick colors that survive dark-mode inversion (avoid pure #000/#fff backgrounds; test text on mid-tone fills).

Deliverability and accessibility:

First element in : a hidden preheader span (~85 chars) that previews next to the subject line.
alt text on any image, lang on , real links (no dead # anchors), and a footer with a plausible unsubscribe line and postal address for anything marketing-shaped.

Show the design at 600px; mention in your reply that the file is send-ready HTML the user can drop into their email tool.

html-email

HTML email