Otdfbt

Vertical spacing is killing me lately. First I had to learn how to get rid of automatically-generated vertical-spacing after center environment because it was off (partopsep+topsep), then I had to learn how to get rid of automatically-generated vertical-spacing after equation or [] environments (set belowdisplayskip and belowdisplayshortskip to 0) because it was off. Now I have difficulties with positioning line of text EVENLY between two horizontal lines drawn with tikz. This is crazy! (could it be more simple, like HTML?)

So I am counting on you to help me out with this.

documentclass[border=5mm,varwidth=150mm]standalone

usepackagetikz

begindocument

 begintikzpicture
 path[draw](0,0)--(textwidth,0);
 endtikzpicture\

 hello\

 begintikzpicture
 path[draw=red](0,0)--(textwidth,0);
 endtikzpicture

enddocument`

Obviously, the text should be slightly lower.

You shouldn't use \ to end a paragraph and the arguably simplest way is to do everything with TikZ.

documentclass[border=5mm,varwidth=150mm]standalone
usepackagetikz
newcounterbpnode
tikzsetevery node/.append style=/utils/exec=stepcounterbpnode,
alias=bpnode-numbervaluebpnode
begindocument
 begintikzpicture 
 node[outer sep=0pt,inner sep=0pt,text width=textwidth,align=left] (txt) 
 hello hello hello hello hello hello hello hello hello
 hello hello hello hello hello hello hello hello hello
 hello hello hello hello hello hello hello hello hello;
 path[draw]([yshift=1em]txt.north west)--++(textwidth,0);
 path[draw=red]([yshift=-1em]txt.south west)--++(textwidth,0);
 %draw (bpnode-1) -- ++ (2,0);
 typeoutthetextwidth
 endtikzpicture
enddocument

Here is a "quick and dirty" proposal using the baseline key.

documentclass[border=5mm,varwidth=150mm]standalone

usepackagetikz

begindocument

begintikzpicture[baseline=-0.6ex]
 path[draw](0,0)--(textwidth,0);
endtikzpicturepar
noindent%
hellopar
noindent%
begintikzpicture[baseline=-0.6ex]
 path[draw=red](0,0)--(textwidth,0);
endtikzpicture

enddocument

Using TikZ, I would put the text in a node of the desired width, then use the node anchors do draw the lines.

documentclass[border=5mm,varwidth=150mm]standalone
usepackagetikz
begindocument
begintikzpicture
node(a)[minimum width=linewidth,text width=linewidth]hello;
draw(a.north west)--(a.north east);
draw[red](a.south west)--(a.south east);
endtikzpicture
enddocument

Here's a non-TikZ answer just to show another approach.

documentclass[border=5mm,varwidth=150mm]standalone
usepackagebooktabs
usepackagecolortbl
usepackagetabularx
begindocument
begintabularxlinewidth@l@
midrule
hello\
arrayrulecolorredmidrule
endtabularx
enddocument

This uses a parbox and vfill. The baselines are ignored in this case, since the baseline of the parbox is more than baselineskip from either above or below, so instead LaTeX adds a gap of lineskip (1pt). Inside the parbox, the vfills expand to the same size. The same effect is achieved by using [c] instead of [s] and vfill, but I wanted to show how it was done.

documentclass[border=5mm,varwidth=150mm]standalone
usepackagetikz
begindocument

 noindentbegintikzpicture
 path[draw](0,0)--(textwidth,0);
 endtikzpicture\
 parbox[c][3baselineskip][s]textwidthvfill
 hello
 vfill\
 begintikzpicture
 path[draw=red](0,0)--(textwidth,0);
 endtikzpicture

enddocument

This uses raisebox to move the text relative to the baseline. The rule shows where the baseline is located.

documentclass[border=5mm,varwidth=150mm]standalone
usepackagetikz
begindocument

 begintikzpicture
 path[draw](0,0)--(textwidth,0);
 endtikzpicture\
 raiseboxdimexpr 0.5depth-0.5heightHellow
 rule1em0.5pt
 raiseboxdimexpr 0.5depth-0.5heightpygmy\
 begintikzpicture
 path[draw=red](0,0)--(textwidth,0);
 endtikzpicture

enddocument

Why use TikZ at all?

documentclassarticle

usepackagexcolor

begindocument

hrule
vspace1ex
noindent
hello
vspace1ex
colorredhrule

enddocument`

As mentioned by others, if you really want to use TikZ you should put the text in a node.

If you want to use this a lot in your document you should of course make it a macro:

documentclassarticle

usepackagexparse
usepackagetikz

NewDocumentCommandmymacroO.5baselineskip O.25baselineskip +m o Ored%
 vspace#2%
 IfValueT#4color#4hrule%
 vspace#1%
 noindent #3%
 vspace#1%
 color#5hrule%
 vspace#2%


NewDocumentCommandmytikzmacroO.5baselineskip O.25baselineskip +m O O%
 parnoindent
 begintikzpicture [line cap=butt]
 node [outer sep=0pt, inner sep=0pt, inner ysep=#1, text width=textwidth-.4pt] (txt) #3;
 draw [#4] (txt.north west) -- (txt.north east);
 draw [red, #5] (txt.south west) -- (txt.south east);
 clip ([yshift=-#2] current bounding box.south west) rectangle ([yshift=#2] current bounding box.north east);
 endtikzpicture%


begindocument

mymacroHello World!

mytikzmacroAdieu World!

enddocument

If what you are trying to do is just putting some frames around text, you might also want to have a look at packages designed for that purpose, like mdframed or tcolorbox.