Technological advances began affecting sentence spacing methods. In 1941, IBM introduced the Executive, a typewriter capable of proportional spacing, which had been used in professional typesetting for hundreds of years. This innovation broke the hold that the monospaced font had on the typewriter, reducing the severity of its mechanical limitations. However, this innovation did not spread throughout the typewriter industry; the majority of mechanical typewriters, including all of the widely-distributed models, remained monospaced, while a small minority of special models carried the innovations. By the 1960s, electronic phototypesetting systems ignored runs of white space in text. This was also true of the World Wide Web, as HTML normally ignores additional spacing, although in 2011 the CSS 2.1 standard officially added an option that can preserve additional spaces. In the 1980s, desktop publishing software provided the average writer with more advanced formatting tools. By the late 20th century, literature on the written word had begun to adjust its guidance on sentence spacing.