For the index page in the subfolder, what is the correct internal link form. 1- a href="folder/index.html" 2- a href="folder/" 3- a href="http://www.domain.com/folder/" 4- a href="http://www.domain.com/folder/index.html" What is the preferred option, and why?
Second and third options are preferred, because shorter URIs are better (for visitors). If you choose second option, use /folder/ instead of folder/ (it solve many issues with relatives URIs) or you can use: <base href="http://yourwebsite.tld"> HTML: Absolute URIs are not recommended, because they increase page size.
The user will see the URL the same in all instances (correct me if I'm wrong), so you shouldn't make a decision based on that. Also, can you please explain the increase in page size (wouldn't the page increase be so small it shouldn't even be noticeable?) I usually go absolute, but I have never read which one to use.
I would choose option #4. Internal linking is important to the search engines, and you won't get a duplicate content penalty because of the 2 URL's for the same page that way.
No, it depends where page is placed (folder, subfolder etc.). You should start to read some HTML tutorial (joking) I am using often absolute URIs too, but when page has a lot of links, than page size is bigger and it results in bad content/source code size ration.
Ok, well let me correct myself. The user would only 2 two different options: http://www.domain.com/folder/index.html /folder/index.html /folder/ http://www.domain.com/folder/ I could see if you have 1,000+ links, but the avaerage page size shouldn't increase.
You have omitted two options. URL starting with folder and /folder are pointing to different places in connection with their placement (folder, subfolder etc. as I already said). I am not sure, what are you suggesting, but I hope you don't want to say that absolute URIs do not increase page size - it is a fact ;-)
I'm going to omit my response to your first section on the basis of not being able to explain on a forum. I think we are thinking different things. I can see how they could increase a page size, but only if you had a lot. A few extra characters on a page will not make a noticeable increase in page size / loading times. Correct?
i think second option is a better option but you must care one more think . the url with / and without / must point the the same url.
It depends on what link you are promoting. If you always use http://www.domain.com/folder/index.html so you shouldn't take the http://www.domain.com/folder/. In my opinion i would take the http://www.domain.com/folder/ and use it all the time and everywhere.