What is no longer necessary when using HTML5 to develop web pages?

Installing a third-party plug-in to include video is no longer necessary when using HTML5 to develop web pages due to the introduction of native support for audio and video elements. HTML5 provides the <video> and <audio> tags, which allow developers to embed media directly into web pages without relying on external plug-ins like Adobe Flash. This built-in functionality not only simplifies the process of adding multimedia content but also enhances compatibility across various devices and browsers, making it a more seamless experience for both developers and users.

The other options involve essential practices and technologies still widely used in web development. Creating responsive designs with CSS is crucial for ensuring websites work well on different screen sizes and devices. Using server-side scripts for dynamic content remains a common approach to generate content based on user interactions or server data. Validating HTML code for errors is important for ensuring the correctness and standards compliance of web pages, thus improving usability and accessibility.