A CSS declaration consists of a property and a value. When a browser’s rendering engine encounters a value it doesn’t recognize or considers unsuitable for the specified property, it treats that declaration as having an incorrect value. For example, setting `width: 15px solid red;` is erroneous because the `width` property expects a length unit or percentage, not a color or line style. The browser will ignore this entire declaration, potentially leading to unexpected layout or styling issues.
Correctly structured style sheets are crucial for consistent cross-browser rendering and predictable webpage behavior. By avoiding incorrect values in CSS declarations, developers ensure the intended styles are applied. Historically, browser inconsistencies in handling incorrect values led to significant development challenges. Modern browsers adhere more closely to web standards, but understanding the underlying principles of valid property values remains essential for robust web development.