The Online HTML Viewer is a web-based development tool that provides a real-time environment for editing and previewing HTML and CSS code. The main interface features a split-screen layout with a code editor on one side and a live preview panel on the other. The editor includes syntax highlighting, auto-completion, and auto-closing tags, while the preview panel instantly renders code changes. Users can import existing HTML files, export their work, and toggle between normal and fullscreen preview modes. The interface is responsive and works across desktop and mobile devices, with a customizable split-view layout that allows users to adjust the size of the editor and preview panels.