Let's break down the provided benchmark and explain what is being tested.

Benchmark Definition

The benchmark defines two different approaches to retrieve the computed style of an HTML element:

1. ownerDocument.defaultView: This approach uses the ownerDocument property to access the default view, which is the top-level window object associated with a document. It then uses the getComputedStyle() method of this view to retrieve the computed style.
2. window.getComputedStyle: This approach directly uses the window.getComputedStyle() method to retrieve the computed style.

Options Compared

The two options are being compared in terms of performance, specifically:

• ownerDocument.defaultView: This option may provide an advantage in certain cases where the window object is not available or is not associated with a valid document. However, it's unclear whether this will be relevant for most use cases.
• window.getComputedStyle: This is a widely used and well-established method for retrieving computed styles.

Pros and Cons

1. ownerDocument.defaultView:
   • Pros: May provide an advantage in certain edge cases or when the window object is not available.
   • Cons: May be slower or less reliable due to the additional check and potential overhead of accessing the owner document.
2. window.getComputedStyle:
   • Pros: Widely supported, well-established, and generally fast.
   • Cons: None notable.

Library/Functionality

The benchmark uses two JavaScript functions:

1. getStyle(): This function is defined in the "Script Preparation Code" section of the benchmark definition. It returns the computed style of an element using either ownerDocument.defaultView or window.getComputedStyle, depending on whether a valid window object is available.
2. getStyle2(): This function is also defined in the "Script Preparation Code" section and simply calls window.getComputedStyle() to retrieve the computed style.

Special JS Feature/Syntax

There are no special JavaScript features or syntax used in this benchmark, only standard ES5 JavaScript.

Other Alternatives

Alternative approaches could include:

1. Using a CSS DOM adapter library like css-dom to provide a standardized API for accessing computed styles.
2. Implementing a custom solution using a combination of DOM and window object manipulation techniques.
3. Using a framework or library that provides a high-level interface for working with styles, such as React or Angular.

Keep in mind that these alternatives would likely introduce additional complexity and potential overhead compared to the simple window.getComputedStyle approach used in this benchmark.