Reducing page weight through the elimination of unnecessary whitespace and comments, commonly known as minimization, and by moving inline script and CSS into external files, can improve download performance with minimal need for other changes in the page structure.
Tables are still considered valid markup, but should be used for displaying tabular data.
To help the browser render your page quicker, you should avoid nesting your tables.
This content is typically text, and can therefore benefit from text compression in transit, thus providing an even quicker response to the user.
Any dynamic features that require the page to complete loading before being used, should be initially disabled, and then only enabled after the page has loaded.
Moreover, valid markup allows for the free use of other tools which can your web pages.
For example, HTML Tidy can remove whitespace and optional ending tags; however, it will refuse to run on a page with serious markup errors.
This will cause the Java Script to be loaded after the page contents, which will improve the overall appearance of the page load.
Inline scripts can be expensive for page loading, since the parser must assume that an inline script could modify the page structure while parsing is in progress.
Do not require your content to appear pixel-perfect in all browsers, especially not in down-version browsers.
Ideally, your basic minimum requirements should be based on the consideration of modern browsers that support the relevant standards.
This may not always be practical; however, you should always take care to use only the minimum necessary number of different domains in your pages.