Adds css era

Added filter for CSS eras

CSS3 (~2009-2012):
A third-level CSS specification, as defined by the CSSWG. (immutable).

CSS4 (~2013-2018):
Basic features that were not part of CSS3, but are already a fundamental part of CSS.

CSS5 (~2019-2024):
New features that are steadily increasing in number.

v3.1.0

Replaces complex viewport_percentage_units_dynamic with “dvb, dvh, dvi, dvmax, dvmin, dvw units”

v3.0.0

Increased the number of CSS items shown by showing CSS values. Currently, instead of 669 items, it has been increased to 2231 😱.

Also increased the number of browsers to be taken into account.

v2.0.0

• switched to Astro
• typing
• abandoned gulp and other dependencies for content generation

v1.0.0

• created a project
• showed css properties, types, at-rule, selectors
• changed the theme