Email HTML is not web HTML
Email clients render HTML with engines that range from a decade behind the web to genuinely idiosyncratic. Outlook on Windows uses Microsoft Word's rendering engine, which has no meaningful support for flexbox, grid, or float-based layout, and handles padding inconsistently.
Consequently, signatures must be built with nested tables and inline styles — the technique the web abandoned twenty years ago. External stylesheets are stripped by most clients, and <style> blocks in the head are removed by several, so every declaration belongs in a style attribute on the element itself.
Use table cell padding rather than CSS margins, specify widths in pixels rather than percentages where precision matters, and avoid background images, which Outlook ignores. Web fonts do not load in most clients, so specify a font stack ending in a system font and accept that many recipients see the fallback.
Images, tracking and the blocked-images case
Most clients block remote images by default until the recipient chooses to load them, so a signature that is entirely one image arrives as an empty box with a broken-image icon. Anything essential — your name, role and contact details — must be live text, not baked into a graphic.
Every image needs meaningful alt text and explicit width and height attributes, so the layout does not collapse when images are blocked. Serve images from a stable, publicly reachable HTTPS URL: signatures are forwarded and archived for years, and a broken image in a signature looks worse than no image.
Keep logos small. A signature adds its weight to every message in a thread, and a large logo repeated through a long conversation is both wasteful and visually noisy. Around 150 pixels wide, at twice that in actual pixels for high-density screens, is generally ample. Note also that Apple Mail's privacy protection and similar features now pre-load images for many recipients, so image loads are not a reliable indication that anyone read the message.
Content, legal requirements and restraint
A signature should carry name, role, organisation and one or two reliable ways to reach you. The common failure is including everything — five phone numbers, six social links, a quotation, a legal disclaimer and an environmental appeal — which buries the useful information and looks unprofessional.
Some elements are legally required in some jurisdictions. UK companies must state the registered company name, registration number, place of registration and registered office address in business emails under the Companies Act; Germany and several other EU states have comparable requirements. Regulated professions often have their own obligations. Confidentiality notices, by contrast, have very little legal effect and are largely ritual.
Consider accessibility and plain text. Colour should not be the only way information is conveyed, contrast should meet the usual thresholds, and a plain-text alternative should carry the same details, since some recipients read mail as text. Test the finished signature in Outlook on Windows, Gmail on the web, and Apple Mail on iOS at minimum — those three cover most recipients and fail in different ways.