The heads of China’s largest Internet and technology firms have vowed to stop the “spread of harmful information” on the web. Web firms came to a “common agreement” to “safeguard” the spreading of positive information online and “strengthen self-management and self-discipline”, the report said. The pledge by the private and state-owned companies backs efforts by the government to tighten its grip on the fast-growing Internet sector, which has become a platform for citizens to express their opinions and frustrations.