看过的许多资料都没有说过htm和html文件有一些区别，所以从来没注意过后缀名的不同有不同的效果。前一阵子做网页，发现有一些标记属性不能在html文件中正确的执行，后来换成htm文件哪些效果就被实现了。上网查过以后才知道他们的区别：
HTML：
html was the original extension for HTML pages on Unix Web hosting machines.
HTM：
htm was created by Windows/DOS because of it's requirement for 3 character extensions.参考网址 ：
http://webdesign.about.com/od/beginningtutorials/a/aa072700a.htmFilenames
HTML documents use the .htm or. html file extensions, e.g. index.htm or index.html can be used to name your opening page. It also a good idea to use lowercase lettering when naming files. In short, a link specifying a file called History.htm will not work if the file is named history.htm, and a link specifying a file called history.html will not work if the file is named history.htm.
Tips:
- HTML tags should be in Upper Case letters to help differentiate from ordinary text.
- All HTML files in your project should be named with either a .htm or .htmlextension. Do not use both extension types in the same project.
- All HTML file names should be in lower case, with letters and numbers only. Do not use spaces or odd characters (%$£&*).
- Backup your Web Site regularly. If you have WinZip or another compression program, you can achieve this by right-clicking on the folder and choosing "Add to Zip". A few seconds later you have a disaster-management and contingency program.
Note: While there are many excellent HTML editors available, it is useful to know enough HTML to code a document before you begin such an editor. The web pages created by such editors may require some "tweaking" of the HTML code in order to work as you would like them to, and it is much easier to do this if you have a grounding in HTML basics.
参考网址 ：http://www.ncte.ie/wpc/start_html.htm