Disable or reset a CSS rule

If define a CSS rule with top: 0;, how can I later disable or neutral开发者_运维技巧ise it in favor of bottom: 0; used elsewhere?

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do top: auto; bottom:0; - auto is the default value for top

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top: auto;
bottom: 0px;

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if you have the stretch issue just use initial to reset the rule

top: initial;

or

bottom: initial;

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CSS is cascading, so for example, if you put p { top: 0;) first and p.bottom {top:auto; bottom: 0;) after that, your css will listen to the latter for p.bottom