Get Firefox the browser, and the self-installing extention called "HTML Validator by TIDY". Both are free of course.
TIDY can check your code and show any errors, and suggest a fix. You have to correct the 'error' in case this makes the page worse (sometimes fixing one error reveals many more!). Most errors however, are "warnings", -minor errors that most browsers still 'understand', but are under no obligation to do so. These are the deprecated (old, outdated & no longer used in best practice), proprietary (generally, IE-only and sometimes, NN ("Netscape")-only), or missing-required end tags as I stated earlier. -Accessibility ommissions are common (not using alt="" for images, title="" for other elements like H-tags, elements of special interest, etc). 10 years ago, this probably made no difference. But 10 years ago, almost nobody was viewing the internet from hand-helds/palm/cellphone/PSPs, etc, -not to mention the issues & legalities of handicapped Accessibility which is becoming expansive to include access to the internet (text-readers/text-to-speech, text-to-braille, etc).
Your page revealed 97(?) warnings, a bulk of which were probably Accessibility warnings (not using summary="summary of the contents of this table" on TABLE tags, required for text-readers to understand what the TABLE is about. Similar to the "chapter title" of chapters in a novel, which 'sum up' or 'catagorize' the contents proceeding it).
Use of descriptive link data... your hyperlinks should for instance never say "click here". A Search Engine does not know how to index "click here", -it is too boolean, like "this" or "that".
A hyperlink should contain a valid "keyword", so if the link points to joes-hardware-store(dot)com, it should be written like "<a href="www.joeshardwarestore(dot)com">Joe's Hardware Store</a> This is "index-able".
And now it's punch-up time for Joe's Hardware Store...
"<a title="Joe's Hardware Store: We sell hardware, power tools, drills, power saws and all accessories for your home improvement needs" href="www.joeshardwarestore(dot)com">Joe's Hardware Store</a>
Okay (-d'ya feel the chill in the air yet?!), -not only did we give the hyperlink a meaningful, index-able text, we gave the SEARCH-engines some title="" to assist people searching for hardware & tools and home improvement! Give yourself 3-points+ for the good hyperlink text, and another 3-points+ for the relevant title="" data.
Another biggie is correctly using <h> tags, which number from 1, 2, 3, -etc to 6.
The FIRST h-tag used, should always be "<h1></h1>". That is the most important. It is called a "first-level header".
The next used h-tag, is "<h2></h2>", -a "second-level header".
The <h3>~, <h4>~, etc.
Try to avoid using say, "<h6></h6>" first just because it is the smallest. You trade presentation for content. Content is KING. Focus on content. If you need a small, "header" that is required at the TOP of a page, you can just go "<h1 style="font-size:1.2em;">This text needs to be small</h1>" which will make the h-1 font be approx. 12 or 14-px high (an "em" unit is about 10-pixels tall).
Avoid ...no, never use the word "banner" to label a DIV or image or logo... "banner" is so maligned with "spam" and "junk mail" that some spider-'bots/search will just consider it bad and give you negative-points right there.
Don't 'sneak in' hidden text keywords, -like very small, repetitive or same-color-as-background text. Most Search engines (google more notably) knows of this 'black hat' technique and can send a site to the back of the list for employing this 'stuffing' technique.
There are dozens upon dozens of 'little things' that can be done to jack-up a sites' ratings. Do the right ones, -don't have to do all of them but just a few biggie ones, -and a site can rise up rather impressively in ranking. Ideally, you would like to rise up to at least the first 10 (the number of sites that usually appear on a single page... most people when searching seldom check or need to check beyong page 1 of any search).
A very important tag... is the <title></title> at the top of the page. A "title" tag is the briefest description of the page's name. Note this page... it probably says "WEBDEVELOPER.COM", not "Welcome to WEBDEVELOPER.COM". You want the most important word, FIRST. So many people start their "<title></title>" with "Welcome to my homepage!" and they loose (get negative bonus points) for this. If the hypothetical page is "Joe's Hardware Store", the title tag should be:
<title>Joe's Hardware Store (and something more if you want...)</title>"
I don't much do META data tags... most SEARCH/'bots ignore them anyway (google especially) due to their rampant abuse in the past. But if you choose to use them anyway (a few sites still glance at them), limit to 56-words or less for keywords="", and do not repeat a word more than 5 or 6 times as this is "stuffing" or 'cramming' the 'bot and they perceive it as "spamming" and again, -bottom of the list for that site. I have seen sites that use meta data like "baseball cards, baseball hats, baseball stuff, baseball, baseball season, baseball penants, baseball memoribilia, baseball hall of fame, etc etc etc " well right there, they just 'spammed' the spider-bot and probably all that imformation is 'ignored' or 'blacklisted'... Spiders go several hundred words 'deep' into a site, seeking index-able content. -Don't 'stuff' the top of the page with useless META data and if you have <STYLE>s that can go to an external file, do it. You 'cleared' that much text out of the path of a hungry spider, allowing it to that much 'deeper' into your site seeking indexable content.
And change. New content is the BEST. Indexers love new content. Even if the content is merely 'revised' a bit, -it's all good. But add a new paragraph and a few more new header tags... spider luv it.
-Like I was saying... hundreds of lil' tricks. Just do the basic, honorable mark-up and you'll be quite pleasantly surprised at how nicely your site can rise up.