Detecting Redundant CSS Rules in HTML5 Applications: A Tree-Rewriting Approach

TL;DR

This paper presents a novel static analysis method based on tree-rewriting to detect redundant CSS rules in HTML5 applications, accounting for dynamic behaviors and improving maintenance and performance.

Contribution

It introduces a formal model of HTML5 applications using monotonic tree-rewriting, analyzes the redundancy problem's complexity, and develops an efficient algorithm for practical detection of redundant CSS rules.

Findings

The redundancy problem is precisely characterized and its complexity established.

An efficient reduction to symbolic pushdown system analysis enables practical detection.

Experimental results demonstrate the algorithm's effectiveness in real HTML5 applications.

Abstract

HTML5 applications normally have a large set of CSS (Cascading Style Sheets) rules for data display. Each CSS rule consists of a node selector (given in an XPath-like query language) and a declaration block (assigning values to selected nodes' display attributes). As web applications evolve, maintaining CSS files can easily become problematic. Some CSS rules will be replaced by new ones, but these obsolete (hence redundant) CSS rules often remain in the applications. Not only does this "bloat" the applications, but it also significantly increases web browsers' processing time. Most works on detecting redundant CSS rules in HTML5 applications do not consider the dynamic behaviors of HTML5 (specified in JavaScript); in fact, the only proposed method that takes these into account is dynamic analysis (a.k.a. testing), which cannot soundly prove redundancy of CSS rules. In this paper, we introduce an abstraction of HTML5 applications based on monotonic tree-rewriting and study its "redundancy problem". We establish the precise complexity of the problem and various subproblems of practical importance (ranging from P to EXP). In particular, our algorithm relies on an efficient reduction to an analysis of symbolic pushdown systems (for which highly optimised solvers are available), which yields a fast method for checking redundancy in practice. We implemented our algorithm and demonstrated its efficacy in detecting redundant CSS rules in HTML5 applications.