The answer to this question, as with several in the world, is “yes and no”. While in English, the word “Web” should be written with a capital “W” in all cases when referring to the “World Wide Web”, other languages address the matter differently. In Spanish, for example, “web” is a term used commonly, but it is at times capitalized and at other times not capitalized. Below are the various cases and the rule for capitalization in Spanish:
The word is capitalized as the term refers to the full name “World Wide Web”. In this same meaning, one can use the Spanish word “Red”, also capitalized.
Adjective in the singular
It is written in lower case, and means that the noun it modifies belongs to the “Red” or the Web. The most common example of this usage is “página web”.
The Pan-Hispanic Dictionary of Doubts also suggests the Spanish words “página electrónica” and “ciberpágina”.
As a noun, the plural is variable and is written in lowercase: “webs”.
Plural of the adjective
As an adjective, it is written in lowercase and is usually invariable: “páginas web”, although the variable form is also recommended: “páginas webs”.
(Versión en español: http://blog-de-traduccion.trustedtranslations.com/web-o-web-2012-03-23.html)