You always optimise FOR something at the expense of something.
And that can, and frequently should, be lean resource consumption, but it can come at a price.
Which might be one or more of:
Accessibility.
Full internationalisation.
Integration paradigms (thinking about how modern web apps bring UI and data elements in from third parties).
Readability/maintainability.
Displays that can actually represent text correctly at any size without relying on font hinting hacks.
All sorts of subtle points around UX.
Economic/business model stuff (megabytes of cookie BS on every web site, looking at you right now.)
Etc.
You always optimise FOR something at the expense of something.
And that can, and frequently should, be lean resource consumption, but it can come at a price.
Which might be one or more of: Accessibility. Full internationalisation. Integration paradigms (thinking about how modern web apps bring UI and data elements in from third parties). Readability/maintainability. Displays that can actually represent text correctly at any size without relying on font hinting hacks. All sorts of subtle points around UX. Economic/business model stuff (megabytes of cookie BS on every web site, looking at you right now.) Etc.