Sitemaps show crawlers where to look when crawling your website.
Provides a list of URLs to crawl through.
This is important because Google ranks web pages and not just websites.
There is no downside of having an XML Sitemap and having one can improve your SEO, so they are highly reccomended.
Structured Data is the information about your page that you want crawlers to view
Sitemaps show when pages were last modified as well as how frequently.
Alt tags help describe images to crawlers
I was surprised that a majority of search engine crawlers still have issues with javascript.
Some search engines also struggle with Flash, though they claim to be better as of late.
Supposedly Ajax also has similar issues to Flash and Javascript.
Another thing to avoid is dynamic urls, urls with a "?" included in them.
Session IDs are another thing to avoid, as it can lead to duplicated content.
The basics in HTML and CSS are generally the most useful for SEO.
Most pages only load the top portion of the page, the part viewers see first.
The rest is loaded as users scroll down the page.
This allows pages to load faster and avoid loading huge images that viewers may not even scroll down to see.
80% of visiters don't scroll down after visiting a page.