What search engines cannot see

It is also worthwhile to review the types of content that search engines cannot “see” in the human sense. For instance, although search engines are able to detect that you are displaying an image, they have little idea what the image is a picture of, except for whatever information you provide them in the alt attribute, as discussed earlier.

They can, however, determine pixel color and, in many instances, determine whether images have pornographic content by how much flesh tone there is in a JPEG image. So, a search engine cannot tell whether an image is a picture of Bart Simpson, a boat, a house, or a tornado. In addition, search engines will not recognize any text rendered in the image. The search engines are experimenting with technologies to use optical character recognition (OCR) to extract text from images, but this technology is not yet in general use within search.

In addition, conventional SEO wisdom has always held that the search engines cannot read Flash files, but this is a little overstated. Search engines are beginning to extract information from Flash, as indicated by the Google announcement at http://googlewebmastercentral.blogspot.com/2008/06/improved-flash-indexing.html.

However, the bottom line is that it’s not easy for search engines to determine what is in Flash. One of the big issues is that even when search engines look inside Flash, they are still looking for textual content, but Flash is a pictorial medium and there is little incentive (other than the search engines) for a designer to implement text inside Flash. All the semantic clues that would be present in HTML text (such as heading tags, boldface text, etc.) are missing too, even when HTML is used in conjunction with Flash.

A third type of content that search engines cannot see is the pictorial aspects of anything contained in Flash, so this aspect of Flash behaves in the same way images do. For example, when text is converted into a vector-based outline (i.e., rendered graphically), the textual information that search engines can read is lost.

Audio and video files are also not easy for search engines to read. As with images, the data is not easy to parse. There are a few exceptions where the search engines can extract some limited data, such as ID3 tags within MP3 files, or enhanced podcasts in AAC format with textual “show notes,” images, and chapter markers embedded. Ultimately, though, a video of a soccer game cannot be distinguished from a video of a forest fire.

Figure 2-21

Search engines also cannot read any content contained within a program. The search engine really needs to find text that is readable by human eyes looking at the source code of a web page, as outlined earlier. It does not help if you can see it when the browser loads a web page—it has to be visible and readable in the source code for that page.

One example of a technology that can present significant human-readable content that the search engines cannot see is AJAX. AJAX is a JavaScript-based method for dynamically rendering content on a web page after retrieving the data from a database, without having to refresh the entire page. This is often used in tools where a visitor to a site can provide some input and the AJAX tool then retrieves and renders the correct content.

The problem arises because the content is retrieved by a script running on the client computer (the user’s machine) only after receiving some input from the user. This can result in many potentially different outputs. In addition, until that input is received the content is not present in the HTML of the page, so the search engines cannot see it.

Similar problems arise with other forms of JavaScript that don’t render the content in the HTML until a user action is taken.

As of HTML 5, a construct known as the embed tag () was created to allow the incorporation of plug-ins into an HTML page. Plug-ins are programs located on the user’s computer, not on the web server of your website. This tag is often used to incorporate movies or audio files into a web page. The tag tells the plug-in where it should look to find the datafile to use. Content included through plug-ins is not visible at all to search engines.

Frames and iframes are methods for incorporating the content from another web page into your web page. Iframes are more commonly used than frames to incorporate content from another website. You can execute an iframe quite simply with code that looks like this:



Frames are typically used to subdivide the content of a publisher’s website, but they can be used to bring in content from other websites, as was done in Figure 2-21 with http://accounting.careerbuilder.com on the Chicago Tribune website.

Figure 2-21 is an example of something that works well to pull in content (provided you have permission to do so) from another site and place it on your own. However, the search engines recognize an iframe or a frame used to pull in another site’s content for what it is, and therefore ignore the content inside the iframe or frame as it is content published by another publisher. In other words, they don’t consider content pulled in from another site as part of the unique content of your web page.