Enhance the online presence of your client's website by expertly crafting and implementing schema markup. This process involves selecting the most relevant schema types tailored to the client's industry and objectives. By adhering to the latest schema.org standards, you will code the schema markup to assist search engines in comprehending the webpage content, thus increasing the likelihood of elevated search rankings. Perform thorough research, create the schema markup code, and utilize the Google Structured Data Testing Tool to validate the code, rectifying any issues detected. Provide comprehensive documentation and clear guidance to your client for managing and modifying the schema markup effectively. The end product should be a robust and validated schema markup that significantly improves the website's visibility and SEO performance.