Here’s an interesting article about naming files for the web: http://www.ragepank.com/articles/50/spaces-underscores-and-dashes-how-to-name-files/
It seems that dashes may be the best way to separate words in a URL. A dash provides better readability than an underscore. Spaces are a bad idea to use because they turn into %20 in a URL and can be misinterpreted by some browsers, be harder to remember, and be harder to communicate verbally.
Here’s another article about spaces in filenames and why they should be avoided in web content: http://www.blackbaudknowhow.com/blackbaud-sphere/spaces-in-filenames-and-why-you-should-avoid-them-on-the-web.htm
Here’s a guide to the different uses of the em dash, the en eash, and the hyphen characters:
A Typography Glossary – covers the language of typography full of serifs, strokes, and swashes.
The All-Inclusive Guide to Web Typography Best Practices – From 8/27/14 – for Bootstrap websites.
5 Typography Do’s and Don’ts Everyone Should Know
Top Ten Web Typography Sins
Underlining for Emphasis?
Underlining in Professional Typesetting?
15 Best Web Safe Fonts
That Work With HTML & CSS – from 5/8/17, by Robert Mening from WebsiteSetup. Arial is one of the most widely used fonts.
Two spaces after a sentence is a holdover from the days before computers and desktop publishing. Typewriters used mono-spaced fonts where every letter had an equal amount of horizontal space. Adding two spaces after a sentence made mono-spaced lines easier to read. However most every font on a PC or Mac uses proportional typesetting so typing two spaces after a sentence are no longer needed yet old typewriter habits still continue. In proportional typesetting skinny letters are given less space than wide ones, white space between letters takes on a balanced appearance. Without the extra white space characteristic in mono-spaced type, breaks between sentences stand out in proportional typesetting without the need for two spaces after a line. Here are some articles with more information on the issue of one or two spaces after a sentence:
American Institute of Graphic Arts (AIGA): http://www.aiga.org/About/
Columbus Society of Communicating Arts (CSCA): http://www.cscarts.org/
International Council of Graphic Design Associations (Icograda): http://www.icograda.org/
Society for Environmental Graphic Design (SEGD): http://www.segd.org/