HTML5 is the latest version of HTML, and it includes a number of new features that make it easier to create dynamic and interactive web pages. Some of these features include support for multimedia elements like video and audio, drag and drop functionality, and improved support for accessibility.

CSS, on the other hand, is a style sheet language that is used to control the presentation of web pages. It includes a wide range of selectors and properties that allow web designers to create unique and custom designs for their websites.

When designing a website using HTML5 and CSS, it's important to consider the overall user experience. This includes not only the visual design of the website, but also the layout and navigation, the load time of the website, and the accessibility of the website for users with disabilities.

To optimize the website for search engines, designers should also focus on using semantic HTML, which is structured in a way that is easy for search engines to understand. This can include using proper heading tags, creating descriptive alt text for images, and using schema markup to provide additional context about the content on the page.

Finally, web designers should also be familiar with responsive design principles, which allow websites to adapt to different screen sizes and devices. This can include using flexible grids and layouts, using responsive images that can adapt to different screen sizes, and using media queries to control the presentation of the website on different devices.