One of the best ways to check your HTML code on all devices is to use a responsive design, which means your website adapts to the device's screen size and orientation. Responsive design uses CSS media queries, flexible layouts, and fluid images to create a dynamic and flexible website. To use a responsive design, you need to set the viewport meta tag in your HTML head, which tells the browser how to scale and render your website. You also need to use relative units, such as percentages, ems, or rems, instead of fixed units, such as pixels, for your layout and typography. Finally, you need to use breakpoints, which are specific points where your website changes its layout or style based on the device's width or height.