Site Validater

Okay, now to see if my test page met the W3C requirements - I knew it was transitional, and I knew there'd be problems. There were problems! Firstly, it had a go at me for leaving out character coding. Okay, fair enough. Then it told me there were missing closing tags - stacks of them. I actually did my test site a couple of weeks ago, but didn't get back to it due to my other studies and another assignment, but I did do a validation and a couple of weeks ago I couldn't figure out why it was telling me I'd left out closing tags when I could see them. Now I know what I have done wrong....I've put my closing tags in the same order as my opening tags. They must be nested....ie, the last open tag is the first closed tag, but I had the closing tags around the other way. Anyway, there were 14 problems - almost all of them due to reversing the closing tags, but also for leaving out the ALT part of image tags, putting # in front of the colour code. Can't say I understand all of what the validator says, but it's a reminder that I will really need to be careful with my code.