This document discusses the challenges of mobile search engine optimization (SEO). It notes that more searches are now performed on mobile than desktop, and users spend most of their time in apps rather than browsers on mobile. For mobile search results, Google aims to provide the most relevant content from both mobile-friendly websites and apps. However, there are challenges such as blocking mobile crawlers using robots.txt, ensuring app and web content match for indexing, and determining whether to build a mobile app or rely on a responsive website. The document provides advice on allowing CSS and JS files to be crawled for mobile, and emphasizes that apps must provide a compelling user experience to stand out from competitors.