Gmail

1. Switch to the HTML Code tab and click Copy HTML
2. In Gmail, go to Settings → See all settings → General → Signature
3. Create a new signature, then click the <> (HTML source) button in the editor
4. Paste the HTML and click OK, then Save Changes

Outlook (Desktop)

1. Copy the HTML code
2. Go to File → Options → Mail → Signatures → New
3. Paste into a plain-text editor, save as signature.htm
4. Place the file in your Outlook signatures folder

Apple Mail

1. Go to Mail → Settings → Signatures
2. Add a new signature, turn off Always match my default message font
3. Use a third-party app like Signature Maker to inject HTML signatures