This is a misunderstanding of the point of Xara Web Designer. Our goal is to provide a tool for designers, not programmers, to have absolute freedom to place anything anywhere, and to offer the first truly accurate WYSIWYG freeform design tool, that delivers consistent, accurate results across all modern browsers. Accurate, predictable text placement is a key part of that. Therefore Web Designer uses pixel accurate placement of all elements on the page including all text lines.
Regarding the use of CSS - see Can I edit the CSS file to change the appearance of my website?
CSS has its place for more technical, less design orientated non-WYSIWYG tools. But it goes against the principles and goals of Xara Web Designer. There is no technical benefit in using CSS files for Web Designer.
Xara Web Designer produces completely compatible HTML, is standards compliant, passes W3C tests, and works on all modern web browsers.