The Case Western HTML Tutorial listed above has a great section on tables . Not all the browsers implement HTML in the same way. Some tags pertain to only one browser. What looks good in the browser of your choice may be unreadable in a different browser. Not supporting a browser can cause millions of people to not be able to see your site.
Some browsers are more forgiving than others.
You trust your browser. If any errors are found in your HTML document, they will be listed by line number with a link to an explanation of the error. Understanding this report can be difficult at times, but stick with it. You should also select the option at the W3C Validation Service to have Weblint check the document - it catches style errors WebTech doesn't. Sometimes ALT tag descriptions aren't needed.