“Element queries are a new way of thinking about responsive web design where the responsive conditions apply to elements on the page instead of the width or height of the browser.
Unlike CSS @media queries, @element Queries are aware of more than just the width and height of the browser, you can write responsive conditions for a number of different situations like how many characters of text or child elements an element contains.
Another concept that element queries brings to CSS is the idea of ‘scoping’ your styles to one element in the same way that JavaScript functions define a new scope for the variables they contain.”

Element Query Conditions

Width-based Conditions

• min-width min-width in px, min-width in %
• max-width max-width in px, max-width in %

Height-based Conditions

• min-height min-height in px, min-height in %
• max-height max-height in px, max-height in %

Count-based Conditions

• min-characters on block elements, on form inputs
• max-characters on block elements, on form inputs
• min-lines min-lines demo
• max-lines max-lines demo
• min-children min-children demo
• max-children max-children demo

Scroll-based Conditions

• min-scroll-y min-scroll-y demo
• max-scroll-y max-scroll-y demo
• min-scroll-x min-scroll-x demo
• max-scroll-x max-scroll-x demo

Aspect-based Conditions

• orientation orientation demo
• min-aspect-ratio min-aspect-ratio demo
• max-aspect-ratio max-aspect-ratio demo

Meta-Selectors

• $this $this demo
• $parent $parent demo
• $root $root demo
• $prev $prev demo
• $next $next demo

Element Queries

Solved by Flexbox is not framework, but a website that showcases how Flexbox makes many CSS hacks obsolete.

“This site is not another CSS framework. Instead, its purpose is to showcase problems once hard or impossible to solve with CSS alone, now made trivially easy with Flexbox. And with the recent release of Internet Explorer 11 and Safari 6.1, the latest Flexbox spec is now supported in every modern browser.”

Check it out