一张网页 or 一个网页 ("a webpage"): which is the better choice of measure word, and why?
A "web page" (网页) is not actually a sheet of paper but a section of a web site (网站). 

On any 'web page' of a site, you can access all other pages within the site,  therefore 个 is the correct classifier for 网页

However, if you print a web page, you'd get "a web page print out" (一张网页的打印输出)