Most computer literate individuals already know that CSS is an abbreviation for cascading style sheets or a term used to described how Hypertext Markup Elements - almost always abbreviated to the initialism HTML - are displayed on computer, tablet, mobile devices, and virtually all other contemporary technological devices.
ALT tags are short for alt attributes or alt descriptions, both of which are technical terms that describe the alt attribute on an image tag - more specifically, a img tag - that lend text characters to be used in search engines.
Most Internet users don't type in the file name of an image they're looking for. Further, when people browsing the World Wide Web and want to find - for example, let's assume they're looking for a web page related to woodworking - something like woodworking, they search one or more terms related to woodworking, then examine the page of results on the search engine they used (Bing, Google, Yahoo, Ask Jeeves, Baidu, etc.) and look for the result that most closely fits their particular interest.
Let's assume that unnamed Internet user that's interested in woodworking is looking for an article with several pictures regarding the setup of a table saw. The user can type in something related to setting up a table saw, obviously increasing their chances of finding something related to a table saw setup.
However, whenever an article uses alt tags - for example, labeling the pictures about table saw setup like this: "table saw setup insert blade," "table saw setup how to steps," and so on - it's more likely the user finds the article with the images using alt tags, rather than an article with images without alt tags.
/* you forgot the 'alt' attribute */
border: 5px dashed #c00;
Adding a red box before the "#c00;" will result in nice red borders around images, alerting you that you forgot to add alt tags.