Definition: A Living Standard is a type of standard, which is continually updated and worked on, and a standard that wont be locked in specific version stages.
Living standards may change over time, though not necessarily the case.
Software developers might not support all specifications in a given standard, instead they might just pick the parts, they find to be most important to their users.
Why Living Standards are better
Version numbering has traditionally suffered from the problem, that the number could potientially end up being very high. This problem is solved by using a Living Standard release model. Version numbering can still be used internally by developers, but should not be a part of the marketing process.
In the case of web browsers, living standards also have the advantage, that individual features can be adopted the moment they are supported by the browsers, and it would be considered valid. With the version numbering of the standards, people would often worry, that features of future versions would be dropped, and they would therefor hasitate to use these features.
Some people had the openion, that it would be best to wait for the W3C to recommend use, before such features chould be used safely. This is however not the case, for multiple reasons. When something has been implemented in browsers, its likely to stay there. Potentially for hundreds of years, to ensure backwards compatibility.
Checking if its safe to use a feature
It will never be safe. There could potentially always be lack of support in hardware and software. But in practice, this is unlikely to happen, because it would lead to loss of market share to solutions with better support for these features.
Generally speaking, a feature is safe to use, when it has been implemented by the parties of interest. In case of web browsers, this would likely be the browsers with the biggest market share, though not always the case. See also: support for old browsers
Examples of Living Standards
HTML is an example of a living standard. The WHATWG maintains the living standard of HTML.