Flash Disadvantages
In terms of usability, Flash sites are similar to sites built in frames, in which visitors cannot bookmark individual pages. This prevents visitors from being able to locate and re-visit specific pages of interest and may prevent a conversion or sale. Also, your visitors may not have the latest version of the Flash plug-in installed or may not have any version at all. This forces your user to download and install software they may not want in order to properly view some or your entire site.
Flash sites are also slow to load, depending on your visitor's connection. Not everyone boasts a lightning-fast internet connection and some Flash-heavy sites can take a minute or more to load!
Flash-based sites are hard on SEO, as they hide the content you want the search engines to see within the Flash file. The more real estate of your website that you have locked up in an animation or Flash movie, the less there is to be found, evaluated and indexed by the search engines.
HTML/CSS Advantages
Using HTML and CSS to build your site is a smart move, in terms of usability. Your content, links, and tags won't be locked up in an animation, as they would be in a Flash presentation, allowing a wider audience to access the information you are offering. Your visitors can bookmark individual pages and return to any page of your website at anytime, leaving them less frustrated and more able to make a purchase easily.
An HTML/CSS coded site also boasts a fast loading time, as much of the code is in separate style sheets. Getting your message across to your visitors in the first five seconds is critical. Your visitors can see your products and services right away, without having to wait for Flash animations or movies to load.
A fast-loading site is also beneficial to your SEO efforts. With much of the code in separate style sheets, your vital site content is much closer to the top of the site code, making it much more accessible to the search engines. With clean code ready for indexing, you'll see better search engines results
Shortly html based is good for seo.