Emails can render differently based on the way that the end user is viewing the email. Content can render differently on Gmail vs Outlook vs Apple Mail. This is particularly true when composing email directly in HTML but there are some best practices you can follow to improve consistency: 

• Use Inline CSS Styles
  Many email clients strip out <style> tags, so applying CSS directly within HTML elements ensures consistent styling. This approach enhances compatibility across different platforms.
• Structure with Tables
  While modern web design favors <div> elements, HTML emails achieve better consistency across clients when using tables for layout. This method helps maintain the intended design, especially in clients like Outlook.
• Avoid Background Images
  Background images may not display properly in all email clients. Instead, use solid background colors to ensure your email looks as intended everywhere
• Ensure Responsiveness
  Design your emails to be responsive, adapting to various screen sizes. A mobile-first approach can be beneficial, ensuring readability and usability on smaller devices.
• Test Across Email Clients
  Before sending, test your email in multiple clients to identify and fix rendering issues. This step helps maintain a consistent appearance for all recipients. When doing advanced testing platforms like Litmus and Email on Acid can be helpful to see how your emails will render.