No matter how well your web site has been constructed it is always worth checking the various attributes for coding errors, 'dead' links, stylesheet errors and much more.
Below are various checks you can perform on your web site (and pages within your website)

HTML Markup Validation

check by W3C enter full url including page name to check.

url: 

check by WDG enter domain name only - checks entire site.

url: 

check by Site Valet enter full url including page name to check.

url: 

visit the Site Valet Page Validation Service

CSS Validation (Cascading Style Sheets)

check by W3C

url: 

visit the W3C CSS Validation Service

Link Checkers

check by W3C

url: 

visit the W3C Link Checker

link check by Site Valet enter full url and page name to check.

url: 

visit the Site Valet Link Validation Service

Spelling Check

check by W3C enter full url and page name to check.

url: 

visit the W3C Spell Checker

Page Evaluation

evaluation by Site Valet enter full url and page name to check.

url: 

visit the Site Valet Evaluation Service