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Sometimes developers will use HTML tables to control the positioning of other items on a web page. This was considered the standard practice before CSS was developed, but many developers still use this methodology today. Although this is a very common practice, the W3C officially discourages it (
Web Content Accessibility Guidelines 1.0), saying "Tables should be used to mark up truly tabular information ("data tables"). Content developers should avoid using them to lay out pages ("layout tables"). Tables for any use also present special problems to users of screen readers." In other words, HTML tables should be used for displaying tabular data on the page, not to build the entire layout of the page. For that, you should use container controls (such as DIVs) and their style attribute, possibly through the use of a separate <style> section or a separate file. This is ideal for a number of reasons:
1· If you use DIVs and a separate stylesheet file to define appearance and position, you won't need to repeat this definition again and again, for each and every page of your site. This leads to a site that is both faster to develop and easier to maintain.
2· The site will load much faster for end users! Remember that the stylesheet file will be downloaded by the client only once, and then loaded from the cache for subsequent requests of pages until it changes on the server. If you define the layout inside the HTML file using tables, the client instead will download the table's layout for every page, and thus it will download more bytes, with the result that downloading the whole page will require a longer time. Typically, a CSS-driven layout can trim the downloaded bytes by up to 50%, and the advantage of this approach becomes immediately evident. Furthermore, this savings has a greater impact on a heavily loaded web server — sending fewer bytes to each user can be multiplied by the number of simultaneous users to determine the total savings on the web server side of the communications.
3· Screen readers, software that can read the text and other content of the page for blind and visually impaired users, have a much more difficult job when tables are used for layout on the page. Therefore, by using a table-free layout, you can increase the accessibility of the site. This is a very important requisite for certain categories of sites, such as those for public administration and government agencies. Few companies are willing to write off entire groups of users over simple matters like this.
4· CSS styles and DIVs provide greater flexibility than tables. You can, for example, have different stylesheet files that define different appearances and positions for the various objects on the page. By switching the linked stylesheet, you can completely change the appearance of the page, without changing anything in the content pages themselves. With dynamic ASP.NET pages, you can even change the stylesheet at run time, and thus easily implement a mechanism that enables end users to choose the styles they prefer. And it's not just a matter of colors and fonts — you can also specify positions for objects in CSS files, and thus have a file that places the menu box on the upper-left corner of the page, and another one that puts it on the bottom-right corner. Because we want to allow users to pick their favorite styles from a list of available themes, this is a particularly important point.
5· CSS enables you to target different classes of devices in some cases without requiring new HTML markup, such as mobile devices like PDAs or smartphones. Due to their constrained screen size, it is necessary to adapt the output for them, so that the content fits the small screen well and is easily readable. You can do this with a specific stylesheet that changes the size and position of some containers (placing them one under the other, rather than in vertical columns), or hide them completely. For example, you might hide the container for the banners, polls, and the header with a big logo. Try to do this if you use tables — it will be much more difficult. You'll have to think about a custom skinning mechanism, and you'll need to write separate pages that define the different layouts available. This is much more work than just writing a new CSS file.
Note : Note that the discussion above referred to the use of tables for the site's overall layout. However, using tables is acceptable to create input forms with a tabular structure, because otherwise too much CSS code would be required in that case to be easy writable and maintainable. It's also not very likely that you'll need to dynamically change the layout of the input form, so you don't need all the flexibility of CSS for that, and using HTML tables is more immediate.
